Emerging trends in direct air capture of CO2: a review of technology options targeting net-zero emissions

The increasing concentration of carbon dioxide (CO2) in the atmosphere has compelled researchers and policymakers to seek urgent solutions to address the current global climate change challenges. In order to keep the global mean temperature at approximately 1.5 °C above the preindustrial era, the world needs increased deployment of negative emission technologies. Among all the negative emissions technologies reported, direct air capture (DAC) is positioned to deliver the needed CO2 removal in the atmosphere. DAC technology is independent of the emissions origin, and the capture machine can be located close to the storage or utilization sites or in a location where renewable energy is abundant or where the price of energy is low-cost. Notwithstanding these inherent qualities, DAC technology still has a few drawbacks that need to be addressed before the technology can be widely deployed. As a result, this review focuses on emerging trends in direct air capture (DAC) of CO2, the main drivers of DAC systems, and the required development for commercialization. The main findings point to undeniable facts that DAC's overall system energy requirement is high, and it is the main bottleneck in DAC commercialization.


Introduction
The increase in the Earth's temperature took a new turn in 1950 and has steadily continued through the 21st century. By the year 2020, however, the global mean temperature was 1.02°C above the preindustrial era. 1 Fig. 1. 6 The most concentrated GHGs in the atmosphere are water vapor and CO 2 and are the leading cause of global warming. In fact, water vapor is more concentrated and can absorb more infrared waves than carbon dioxide. 8 The global temperature increases due to anthropogenic activities, especially when humans use fossil fuels that emit CO 2 and with higher CO 2 concentrations, the atmosphere gets hotter and more humid. 9 Thus, reducing the CO 2 emissions in the atmosphere will result in low water vapor concentration and reduced global mean temperature. 10,11 Preventive and remediation methods can reduce CO 2 emissions. The preventive approach includes using renewable energy systems and improving systems efficiency, 12 while the remediation includes capturing, storing, and utilizing CO 2 . 13 Although the whole world exerts efforts to reduce CO 2 emission by preventive methods, billions of tons of CO 2 is still being emitted into the atmosphere. Considering the continuous emissions, IPCC proposed CO 2 capture as a necessary.
There are two types of CO 2 capture technologies: the conventional stationary sources 14 and the direct air capture (which removes CO 2 directly from the atmosphere). The conventional CO 2 capture technology prevents the emitted GHGs from spreading to the atmosphere. 15,16 As shown in Fig. 2, the conventional CO 2 capture technique is subdivided technology to limit CO 2 emissions 17 into pre-combustion, oxy-fuel combustion, and post-combustion capture. 18,19 In precombustion carbon capture, a production of syngas (hydrogen and carbon monoxide mixture) from fuel reforming is followed by CO 2 separation process. 20 The oxy-fuel combustion technology includes fossil fuel burning in the presence of pure oxygen. 21 The post-combustion approach involves capturing CO 2 from ue gas (end of pipe treatment approach). 16,22 The three mentioned conventional carbon capture techniques prevent emissions of CO 2 to the atmosphere from burning of fossil-based fuels. On the other hand, Negative Emissions Technologies (NETs) including direct air capture technology (DAC) creates an outlet to directly capture the existing CO 2 from the atmosphere to achieve negative emission goals. 23 Although  the IPCC proclaimed that fossil fuel usage was needed to be immediately reduced in 1990, 24 fossil fuel global primary energy consumption represented 84.3% in 2019. 25 As a result, the latest report from IPCC stated that the contribution of NETs are required to stabilize the CO 2 emission at double the preindustrial levels by the second half of 21st century. 26 Moreover, the need for rapid deployment of negative carbon technologies were proposed by Paris talks and Nation Research Council (NRC) to meet the 2°C limit and net-zero carbon in the later part of 21st century goals. 27,28 Negative emissions technologies (NETs) are carbon dioxide removal (CDR) techniques from the atmosphere, such as direct air capture (when carbon removal is achieved through physicochemical processes such as adsorption, absorption, ocean alkalinity enhancement and soil mineralization) or indirect air capture (when carbon removal is achieved through biological processes such as bioenergy with carbon capture and storage (BECCS), ocean fertilization, afforestation, biochar and algae culture). 29,30 Under direct air capture, there is adsorption, which is the capture of CO 2 in the pores of solid sorbents (physisorption) 31,32 or the reaction of acidic natured CO 2 with basic sites on solid materials (chemisorption). 32,33 Alternatively, CO 2 can be captured in the liquid volume in the process known as absorption. In absorption, reactive compounds (such as NaOH, Ca(OH) 2 , KOH and amines) are dissolved in liquid phase, which allows CO 2 from air to be retained within the liquid volume. 34 Ocean alkalinity enhancement is an absorption process in which CO 2 is absorbed in ocean water and reacts with ocean minerals to neutralize ocean acidication. 35,36 This process alone captures about one-third of the global CO 2 emissions from the atmosphere. 34 Soil mineralization is another direct air capture approach, it is the largest land-based carbon sink on the planet and it is capable of capturing about 3 GtC year −1 . 37 Soil carbon mineralization follows three approaches, 38 (i) buildup of organic carbon as a result of plant growth; (ii) rock weathering (helping the breakdown of inorganic carbon in soil solution); and (iii) precipitation of carbonate materials. However, ocean alkalinity enhancement and soil mineralization were rarely investigated in the literature and more studies are required to show if the technology can be used at scale. 39 Detailed pictorial representations of NETs are shown in Fig. 3 below. BECCS is the most researched NETs under indirect air capture. 40 CO 2 from the atmosphere is captured naturally by microorganisms and plants through photosynthesis, thereby producing biomass. Since biomass is considered a clean source of energy, the biomass obtained can be used to generate electricity via thermoelectric power plant, thereby translating to negative emissions, 34,42 however, its use is dependent on the availability of biomass, land and storage. 43 Afforestation is another indirect air capture that has the capacity to store large amount of CO 2 . Just like soil mineralization, afforestation is a technique that can be deployed anytime because it is readily available. 44 Planting new trees or better management of the existing forests can increase the natural rates of carbon capture from the atmosphere, 45 although it has been reported in the literature that this may lead to loss of biodiversity, 46 loss of valuable land for crops, 47 and increase in loss of ice and local warming in high altitude forest. 48 Ocean fertilization increases biomass productivity on the ocean oor when micronutrients (such as iron) and macronutrients (such as phosphorus and nitrogen) are dissolved into the ocean, whereby the expected microorganism sinks to the ocean oor. 49 Algae (microalgae and seaweeds) culture has been reported to have photosynthetic efficiency that is ten times higher than plants. 50 Microalgae culture has the capacity to capture CO 2 from different sources, such as from ue gas point sources and distributed sources like the atmosphere. 50 The most remarkable part of CO 2 biosequestration procedure is the recycling of part of biomass in the form of biochar. Biochar is produced through pyrolysis or gasication of biomass. 51 Currently, biomass remains are burned or decomposed in soil; and as a result, large amount of the CO 2 captured through photosynthesis is emitted to the atmosphere. When biomass is converted to biochar for soil nourishment, CO 2 emission may be reduced by as much as 1.8 Gt year −1 without CCS. 52 About half of CO 2 yearly emissions are from distributed sources. The fact that DAC systems can remove CO 2 from both distributed and point sources, are not location specic, do not have contamination issues such as (NO x and SO x ), and has small footprint, shows the necessity of deploying DAC over point sources and other NETs. 53 However, capturing CO 2 from dilute air is an energy intensive process. The minimum CO 2 separation energy required in case of dilute air (400 ppmCO 2 ) was calculated to be about 20 kJ per molCO 2 , 16,[29][30][31] while the minimum energy required to capture from ue gas using benchmark aqueous MEA is 8.4 kJ per molCO 2 . 54 DAC is a relatively new technology that is still in its early commercial stages. The early startups that have contributed immensely to DAC commercialization are Carbon Engineering in Canada, Climeworks in Switzerland and Global Thermostat in the United States. 55 Different DAC studies have shown that chemisorption is more relevant to DAC than physisorption (activated carbon, zeolites, and metal-organic frameworks) because physisorption has poor performance in capturing CO 2 at low concentration streams in the presence of water vapor. 56,57 The high energy requirements of DAC lead to high capital and operating costs, which is a major challenge of the existing technologies. 58 The costs are reported in the literature for three main types of DAC systems, which are high temperature aqueous solutions, 59-64 low temperature solid sorbents, [65][66][67] and moisture swing solid sorbents. 68 However, these reported costs were not comparable due to different assumptions and outputs. Fasihi et al. 69 recalculated these costs based on xed assumption and reported cost range of 115-388, 120-244 and 99 EUR per ton CO 2 for high temperature aqueous solutions, low temperature solid sorbents and moisture swing solid sorbents, respectively. A DAC cost of Gigaton scale at less than $100 per ton CO 2 by 2050 is needed for the technology to achieve great climate impact, which highlights the need for cost reduction in DAC technologies. 70 Many studies have been carried out to estimate the potential cost of DAC to show if it would be a climate change valuable solution in the future. Simon et al. 71 examined a generic DAC technology, and claimed that capturing CO 2 at 220 EUR per ton CO 2 is currently possible but indicated that the calculated cost could be provided by more research into kinetics and capturing thermodynamics. Moreover, the authors estimated a range of cost from 75 to 800 EUR per ton CO 2 based on pessimistic and optimistic scenarios. House et al. 72 pointed out that the literature underestimated the cost of DAC, and the current cost of DAC is as high as 750 EUR per ton CO 2 , however, the cost has the potential to reach 225 EUR per ton CO 2 with a technological breakthrough. In fact, most of the studies in the literature show that DAC technology cost will be reduced with time. The cost of capturing CO 2 was estimated to reach 30, 71 and 105 EUR per ton CO 2 based on optimistic, realistic, and pessimistic scenarios by Broehm et al. 73 Nemet and Brandt 74 also estimated a low DAC cost of 45, 23 and 14 EUR per ton CO 2 by 2029, 2050 and 2100, respectively based on the assumed 10% learning rate and a lifetime of 50 years, which is higher than other assumptions in the literature by 20 years. Mahdi et al. 69 proposed that the line of research should be more intensive on low temperature solid sorbent compared to high temperature aqueous solutions because it has higher potentials for cost reduction to reach 54 EUR per ton CO 2 for low temperature systems compared to 71 EUR per ton CO 2 for high temperature system by 2050 based on 15% learning curve. The reason for this potential is the fact that low temperature solid sorbents can use low thermal grade heat, does not required external water and has high modularity.
In the recent years, considerable efforts have been made by carbon capture researchers to collate work on direct air capture of CO 2 in the form of review articles. Several review articles have been published on DAC sorbent materials and technology options. [75][76][77][78][79][80] For example, the work of Deng et al., 75 Zhu et al., 76 Cherevotan et al., 77 and Shi et al. 78 focused on various forms of sorbent materials for DAC applications, while McQueen et al. 79 provided analysis on current as well as future DAC technologies, and Erans et al. 80 assessed DAC technologies with a focus on techno-economic and socio-political challenges. A search through the literature showed few review articles that combine all the highlighted elements in a single publication. This work, nonetheless, combines the review of DAC sorbent systems with technology options. Moreover, to the best of our knowledge, this work is the rst article to review DAC integration with HVAC systemsan emerging technological option.
As a comprehensive review, the article starts by highlighting the importance of using porous material to capture CO 2 from the atmosphere over any other process. All the absorption and adsorption-based technologies are assessed in detail with tables to allow for the comparison. A general overview on the commercialization of DAC is presented, and for the rst time, a detailed review of the articles that touch on DAC-HVAC systems' integration is carried out. The work highlights the main challenges and benets of the integration, presents mathematical models for the integration evaluation, proposes certain suitable technologies based on the conducted reviews and shows a brief technoeconomic study of HVAC-DAC unit integration to demonstrate the cost benets therein.

Description of DAC
The direct air capture (DAC) system setup consists of sorbents, contact area, and regeneration module. The sorbent is a liquid or solid material that attracts CO 2 either chemically or physically. For CO 2 capture to occur, ambient air is exposed to the sorbent material through the contact area, and upon saturation with CO 2 or as desired, the sorbent material undergoes regeneration in which it is separated from the captured CO 2 to get concentrated CO 2 stream. The sorbent material should be reversible so that it can be used many times. The removal of CO 2 from ultra-diluted air by heating, cooling, air compression or using membranes requires an extremely high-energy consumption. 78 For example, 2.2 MJ mol −1 of CO 2 of air-cooling is required to form dry ice and 7 MJ per mole of CO 2 is required for 1 bar pressure drop through membrane separation. 81 On the other hand, binding CO 2 with sorbent materials consumes little or no energy, but the most energy required in the process is associated with removing high concentrated CO 2 from the sorbents. 78 In the literature, sorbents associated with DAC technology are classied into two, liquid 68 and solid 82 sorbents. Solid sorbents do not lose heat to evaporation as liquids; it has better kinetics and is more effective in preventing the loss of volatiles to the atmosphere. 83 The liquid (absorption-based systems) and solid (adsorption-based systems) sorbents used in DAC systems are described below.

Absorption based systems
Absorption is a technique used to remove CO 2 from a gas stream into the bulk of liquid sorbent based on chemical or physical interactions. The absorption of CO 2 can be classied into the absorbent types such as alkanolamines absorption, 84 dual alkali absorption, 85 aqueous ammonia absorption, 86 sodium carbonate slurry absorption and chilled ammonia absorption. The absorption processes used in DAC systems are limited to the chemical sorbents with strong CO 2 binding affinities. 78 The following will include only the absorbents that are used in DAC systems.
Calcium hydroxide solution. Calcium hydroxide solution (Ca(OH) 2 ) has strong binding energy to CO 2 , so a passive or agitated pool of the solution is used to capture CO 2 by precipitating calcium carbonate (CaCO 3 ). 53 The separation of carbon dioxide (CO 2 ) from CaCO 3 required both drying and CO 2 regeneration in a process called calcination at 700°C or higher temperature. Finally, the Ca(OH) 2 is reproduced by reacting the calcium oxide (CaO) with water in a process called slaking. The reactions involved in the cycle are as follow: Ca(OH) 2 + CO 2 / CaCO 3 + H 2 DH˚= −109 kJ mol −1 (1) The use of Ca(OH) 2 solution was implemented in DAC technology by Lackner in 1999. The energy consumed for the calcination process was 179.2 kJ per mole of CO 2 , which is higher than the minimum thermodynamic calcination energy of 109.4 kJ per mole of CO 2 . 81 The main issues associated with Ca(OH) 2 are that it consumes even more energy in drying, and it has limited concentration due to the very low solubility of calcium hydroxide in water. 81 Sodium hydroxide solution. Sodium hydroxide (NaOH) has been used in the industry since 1884 in the kra process to remove cellulose from woods. The same principle is used to capture CO 2 and to regenerate NaOH. 87 NaOH offers strong binding to CO 2 like Ca(OH) 2 with the advantage that the formed carbonate is highly soluble in water, so build-up (scaling) on the inner surfaces of the absorption column is avoided; however, the high solubility of sodium carbonate prevents direct precipitation. 88 Separation of sodium carbonate from the solution requires high energy to evaporate a large amount of water and yield sodium carbonate. Instead, CaCO 3 and NaOH are produced by reacting sodium carbonate with Ca(OH) 2 in a process called causticization. 59 The concentration of NaOH solution in causticization process is limited to 1 mole L −1 because at higher concentrations undesired Ca(OH) 2 will be formed. The carbonate ions are exchanged between calcium and sodium with a theoretical efficiency of 96%. 54 Finally, the calcination and slaking processes used in the case of Ca(OH) 2 are also used in the kra process to separate the CO 2 stream and to regenerate the Ca(OH) 2 solution as in Fig. 4 From the literature, DAC based kra process systems decompose CaCO 3 to release CO 2 stream at a temperature of 900°C and the range of energy required is 1420-2250 kW h per ton of CO 2 .
Potassium hydroxide solution. Potassium hydroxide (KOH) is also considered an effective CO 2 capture sorbent material that can replace NaOH using the same mentioned technique. The use of KOH was proposed as a non-toxic solution, and the results of experimental laboratory work for improved contactors were investigated. 61 One study proposed an alternative technique in which sulfuric acid reacts with potassium carbonate to form CO 2 and potassium sulfate (K 2 SO 4 ). For completing the cycle, electrolysis with a cation exchange membrane is used to regenerate both KOH and sulfuric acid (H 2 SO 4 ). 90 Other alternative absorption processes. The drawbacks, such as the high-energy demand, 1 molar concentration limit, and the low efficiency associated with using Ca(OH) 2 in the kra process as a second absorbent inspired scientists to explore other processes in the industry. From the pulp industry, sodium metaborate was proposed to be used in the CO 2 capture cycle instead of Ca(OH) 2 as shown in the reactions below. It was demonstrated that borates could replace the lime in the kra process to increase the amount of caustic available. 90 However, the process still requires high energy and works at high temperatures, 900°C and above. NaBO 2 + Na 2 CO 3 / Na 3 BO 3 + CO 2 (8) Another method to directly causticize sodium carbonate without using lime is the decarbonization reaction between sodium carbonate and Na 2 O$3TiO 2 . The overall reaction enthalpy is only 90 kJ mol −1 , which is half the energy of causticization with lime (179 kJ mol −1 ). 91 However, the reaction requires a high temperature above 800°C. The CO 2 capture cycle reactions using the mentioned technique is presented below: The CO 2 absorbed by NaOH is experimentally released using H 2 -recycling electrochemical cell. Using the cell, a pH gradient is created so that at low pH CO 2 is released, while at high pH NaOH is regenerated. This approach regeneration energy is 374 kJ per molCO 2 . 92 Sabatino et al. 93 conducted a techno-economic study for a DAC system that uses aqueous KOH solution for absorbing CO 2 from air while the regeneration is implemented using bipolar membrane electrodialysis (BPMED). The study showed that the regeneration energy required is 236 kJ per molCO 2 (1491 kW h per ton CO 2 )and the total cost of capture is $773 per ton CO 2 , which is still higher energy-consuming process compared to Carbon Engineering system. 87 Other absorbents are aqueous amines, which are well known materials that efficiently absorb CO 2 from point sources with CO 2 concentration ranging between 12-15% v/v. In the context of the binding step, aqueous solutions of NaOH or KOH are preferred over liquid amines (MEA) because NaOH or KOH solutions have faster kinetics and are stronger bases than liquid amines. NaOH has a larger capacity and can efficiently bind to a mole of CO 2 per mole of NaOH, whereas MEA binds efficiently only to a half mole of CO 2 per mole of MEA. 60 In the context of regeneration, amines have a low heat of adsorption, 90 kJ per molCO 2 , compared to the traditional kra process, 179 kJ per molCO 2 ; moreover, amines can be regenerated at a lowtemperature range; between 100-120°C compared to calcination process which needs a temperature of 900°C. The nature of capturing CO 2 from the atmosphere (diluted source) is energyintensive, motivating researchers to investigate different liquid amines with better CO 2 uptake performance as amines have the potential for lower energy consumption. 94 Hanusch et al. 95 Investigated the CO 2 uptake performance of 6 pyrrolizidine derivatives compared to MEA and linear amines. Sample number 8 (N-methyl-1-(tetrahydro-1H-pyrrolizin-7a(5H)-yl) methanamine) reached 90% of its capacity 3 times faster than MEA, highlighting the advantage of pyrrolizidine diamines caged structure. Moreover, the experiment showed that sample 8 achieved an absorption capacity of 1.06 molCO 2 /mole Amine , and it was experimentally stable for 14 cycles under pure CO 2 absorption. For absorption from diluted air (400 ppm CO 2 ), sample 6 (5-aminomethyl-1-azabicyclo[3.3.0]octane) showed no oxidation. Barzagli et al. 96 screened the performance of different amines types (primary, secondary, and tertiary) and compared them to aqueous NaOH, sodium carbonate, and potassium glycinate based on a xed concentration of 1.5 mol dm −3 . The study shows that MEA, DGA, 1A2P, 2A1B, MMEA, EMEA, and BUMEA had CO 2 uptake values close to NaOH and potassium glycinate based on 1 and 24 hours experiments. Other issues associated with using aqueous amines are the high energy consumed in water evaporation and high volatility and toxicity, which motivate the researchers to use organic dilutants. Vapor pressure and the higher solubility of CO 2 in organic dilutants compared to water was reported to lead to lower regeneration energy and lower desorption temperatures. 97 However, the aqueous amines reported capacities were higher than the same amines in organic diluents. 96 Other liquids used to capture CO 2 in air are aqueous amino acid solutions, which are nontoxic, nonvolatile, non-corrosive and environmentally friendly. Brethomé et al. 82 reported a proof of concept that uses amino acid to absorb CO 2 from air, and then the CO 2 -loaded amino acid reacts with guanidine compound. The result is the crystallization of a guanidinium carbonate salt, which has limited solubility and easy to be separated from the amino acid. The nal step is to regenerate CO 2 from the guanidinum carbonate salt by low thermal grade heating as shown in Fig. 5. Even with all the mentioned advantages of the indicated cycle, the equilibrium capacity and cyclic capacity of 1 M aqueous potassium glycinate solutions was reported to be only 0.76 and 0.28 molCO 2 /mol aminoacid , respectively. While the regeneration energy reported is 223 kJ per molCO 2, with a large part of that energy consumed on water evaporation. If there was no water before the CO 2 release, the regeneration energy could be reduced to 75 kJ per molCO 2 . Cai et al. 98 used trichelating iminoguanidine ligand (BTIG) to bind CO 2 from air, forming insoluble crystal, which can be easily separated from the solution, an equilibrium capacity of 0.99 molCO 2 /mol BTIG was achieved. The BTIG can be regenerated aer absorbing CO 2 by mild heating (100-150°C), with decomposition energy consumption of 169 kJ per molCO 2 including the heat consumed in water evaporation. It was indicated that the slow kinetics of crystallization of BTIG-CO 2 can be two times faster by adding glycine.
Absorption unit limitations and novel designs. Due to the large volume of air treated by a direct air capture plant, the pressure drop is associated with a large energy penalty. Towers with lled packing materials are commonly used in the industry for absorbing a gas into solutions. The solution is dropped from the upper end of the towers while the gas is blown from the bottom. Different air contactor designs were assessed to be used for CO 2 capture, but in the case of dilute air, large cross-section and short columns were reported to be more efficient. 59 An example of the need for large cross sections columns to capture CO 2 from dilute air is the study done by Keith et al. 59 A capture unit in the study was designed to have an inlet and outlet CO 2 concentration of 500 ppm and 250 ppm, respectively. The absorbent used is 1 M hydroxide sodium with a 1.44 liquid to gas ratio. The absorber was designed to allow for a pressure drop of 100 Pa m −1 , resulting in a column with 2.8 m and 12 m in height and width, respectively. 59 In order to reduce the pressure drop, the spray towers were proposed to be used instead of open pools and packed towers. The spray tower offers a larger surface area, reduced pressure drop, and lower construction costs compared to other designs, but also has its own energy penalty. 82 The study 82 also demonstrated how spray towers can reduce the air with 450 ppm CO 2 concentration by 34 ppm, which is equivalent to absorbing 7.4 mmol L −1 of solution/pass. Moreover, to capture 1 ton of CO 2 , 1638 tons of atmospheric air is required, which implies large energy consumption by fans. 87 Using the integration between DAC and natural dra from natural dra dry cooling towers (NDDCTs), the air pressure drop and fans energy penalty were avoided using the cost-free natural dra from NDDCTs instead of mechanical fans. 99 A large portion of energy is also detected in removing moisture from calcium carbonate before the calcination process by a kiln. The average mass of water loss was reported to be 90 g of H 2 O per g of CO 2 captured. 34 One other limitation that increases the energy penalty is that the kiln uses oxygen instead of air to avoid CO 2 separation from nitrogen. The whole calcination process highly consumes energy as it contributes to 4.5 GJ t −1 of CO 2 . 88 In the case of spray towers, the water losses by evaporation were tested for various NaOH concentrations. The results showed that reaching a high NaOH solution concentration of 7 M could eliminate the water losses. 82 One primary concern with spray towers is the coalescence that increases for higher ow rates and longer air contactors; however, the reduction in ow rate leads directly to a lower CO 2 absorption rate. 82 The proposed integration between DAC and natural dra dry cooling towers (NDDCTs) allows harvesting the consumed energy in evaporating water for improving the cooling process. The study shows that harvesting the energy increases the cooling system efficiency by 16%. 99 The main parameters of different DAC systems based on solvents are illustrated in Table  1, and the system with the least energy consumption is associated with the use of Na 2 O$3TiO 2 (ref. 91).

Adsorption based systems
Adsorption is a reversible process where the solid adsorbent captures the molecules, atoms or ions of gases and liquids on its surface by physical means like van der Waals forces or by forming chemical bonding. The reversibility of adsorption process is a function of temperature and pressure, which means the adsorption and desorption capability can vary by varying both temperature and pressure. CO 2 can be adsorbed at high pressure and then released when the pressure is lowered in a process known as pressure swing adsorption (PSA). The same process could be implemented by alternating the temperature in a process called temperature swing adsorption (TSA). 16 The solid adsorbents will be divided in the present work among the three main categories, which are physisorption, chemisorption, and moisture swing adsorption. In chemisorption, CO 2 binds strongly to the adsorbent by chemical bonding, which has the capacity to capture CO 2 even in ambient air. However, it needs high energy to release the CO 2 from the sorbents through heating like what occurs in the calcination process. 102 On the other hand, the physisorption requires less regeneration energy but it has low CO 2 selectivity and capacity for atmospheric concentrations. 56 Another process that does not consume much energy in releasing CO 2 is the electrochemical CO 2 capture, but it has been reported that it is only effective if CO 2 concentration falls between 15% and 30%. 102 Finally, moisture swing adsorption combines the advantages from both chemisorption and physisorption, as it offers a high selectivity and capacity with low regeneration energy. In the moisture swing adsorption, CO 2 can bind to the sorbent materials if it is dry, while the separation occurs if the material is wet. The energy consumption in the process is associated with water evaporation for drying the sorbent material. 103 Different physisorption, chemisorption and moisture swing sorption materials will be discussed in the later sections.
Physisorption. In physisorption, CO 2 physically binds to the surface of the sorbent materials using interaction forces such as van der Waals forces. The evaluation of effective CO 2 adsorbent depends on selectivity, capacity, adsorption & desorption rate, stability, and adsorption & desorption temperature. Capacity and kinetics of adsorption are affected by both adsorbent chemical composition and structure, while the CO 2 separation mainly depends on adsorption kinetics, difference in equilibrium concentration and molecular sieving mechanisms. Although physisorption requires less energy compared to chemisorption, the usage of physisorption in CO 2 capture has capacity and selectivity limitations. 56 There are various materials used in physisorption that can be classied based on their pore sizes (microporous, mesoporous and amorphous) or on their chemical composition such as zeolites, activated carbon and metal organic frameworks. Based on the literature, 53,56,57 chemisorption materials are more relevant to DAC compared to physisorption materials. As a result, the present work will give a quick look at different physisorption materials that were used in DAC and their potentials and limitations.
Zeolites. Zeolites are materials that compose of aluminum, oxygen, and silicon; they are microporous crystalline materials with matrix channel cavities and high porous surface area as shown in Fig. 6. Adsorption using zeolite is desirable in the temperature range between 0°C and 100°C and pressure range between 0.1 to 1 bar of CO 2 pressures. The adsorption capacity of zeolites decreases as the temperature increases, so it is preferred in cold streams. A negative charge is induced from the silicate cations, which results in the capacity of zeolite to adsorb different ue gas components. It was reported that increasing the K + /(K + + Na + ) ratio in zeolites until 17% can make adsorption of CO 2 high compared to a negligible adsorption of N 2 . 104 The adsorption capacity of different commercial zeolite materials (APG-II, WE-G 592, 13X, 5A, and 4A) was compared, and it was reported that 13X has the highest adsorption capacity. The hydrophilic nature of zeolite, which causes strong water adsorption usually leads to low selectivity of CO 2 in direct air capture amidst atmospheric air constituents. It was reported that increasing the K + /(K + + Na + ) ratio in zeolites until 17% can make adsorption of CO 2 high compared to a negligible adsorption of N 2 . 104 The adsorption capacity of different commercial zeolite materials (APG-II, WE-G 592, 13X, 5A, and 4A) was compared, and it was reported that 13X has the highest adsorption capacity. The hydrophilic nature of zeolite, which causes strong water adsorption usually leads to low selectivity of CO 2 in direct air capture amidst atmospheric air constituents. 22 MOFs. Metal organic framework (MOFs) are organic and inorganic materials with crystalline pores. These materials compose of organic molecules that surrounds positive metal ions. MOFs have large surface area, low density, and porosities up to 6000 m 3 g −1 , which make MOFs a potential adsorbent, 105 however, MOFs have poor adsorption capacity for low CO 2 partial pressure compared to zeolite and activated carbon (0.1-0.2 bar). 106 MOFs adsorption from the ambient air requires very high CO 2 partial pressure, moreover, adsorption selectivity is negatively affected by the presence of moisture in air. 107 A recent study designed a DAC system using MOFs-polymer nanocomposite. The study reported CO 2 purity between 70% and 80% with high water vapor repellence. 107 compared to zeolite and activated carbon (0.1-0.2 bar). 106 MOFs adsorption from the ambient air requires very high CO 2 partial pressure, moreover, adsorption selectivity is negatively affected by the presence of moisture in air. 107 A recent study designed a DAC system using MOFs-polymer nanocomposite. The study reported CO 2 purity between 70% and 80% with high water vapor repellence. 107 Activated carbon. Activated carbon is a charcoal that is puried and powdered then chemically or physically treated to create micro ssures that increase adsorptive surface area. It's adsorptivity is effective because of the large surface area (500-1500 m 2 g −1 ) and electrical charge. Activated carbon is prepared from two steps, which are carbonization and activation, and it is characterized by thermal stability and low cost. Activated carbon has a lower capacity and selectivity compared to zeolites for low CO 2 partial pressure but in higher pressure, activated carbon exhibits a higher capacity. These differences in the adsorption characteristics associated with pressure changes make activated carbon work well with pressure swing adsorption (PSA). Activated carbon selectivity towards CO 2 in dilute air is low because of the existing moisture, but researchers are investigating different ways to improve its capacity and selectivity, but researchers are investigating different ways to improve its capacity and selectivity (Fig. 7). 78 DAC relevancy summary. Although the physisorption materials consume less energy in regeneration compared to chemisorption materials, these materials have low CO 2 capacity and selectivity in the case of atmospheric CO 2 levels because of the existing moisture. Table 2 shows different physisorption materials that are used in DAC systems with focus on the selectivity of CO 2 compared to water vapor. Four different physisorption sorbent materials were studied using temperature programmed desorption (TPD) and the results show that these materials have poor selectivity towards CO 2 while the chemisorption material (TEPA-SBA-15) show much higher selectivity towards CO 2 . 108 Other 10 physisorption materials were tested using the same technique and the results are shown in Table 2. 109 Although the required energy in physisorption is low, the assessment of different physisorption materials show that the materials have very low selectivity towards CO 2 compared to water vapor, which make their usage in DAC system unnecessary. 108,109 Zeolite 13X was proposed for capturing CO 2 from air, yet the humidity and high temperature negatively affected the zeolites. Only MOFpolymer nanocomposite shows a high selectivity towards CO 2 as it can produce a stream with CO 2 purity of 70-80% in presence of water vapor. 107 The use of boron nitride nanotube and nanosheets was also proposed to be a strong CO 2 adsorbent with high selectivity and reversibility. The materials simply adsorb CO 2 by introducing electrons to the adsorbent and then release CO 2 by removing the electrons. 109,110 Although the  Ref.
a Mass of analyte in mg g −1 in parenthesis. b Produced CO 2 purity. c A chemosorption material for comparison. required energy in physisorption is low, the assessment of different physisorption materials show that the materials have very low selectivity towards CO 2 compared to water vapor, which make their usage in DAC system unnecessary. 108,109 Zeolite 13X was proposed for capturing CO 2 from air, yet the humidity and high temperature negatively affected the zeolites. Only MOFpolymer nanocomposite shows a high selectivity towards CO 2 as it can produce a stream with CO 2 purity of 70-80% in presence of water vapor. 107 The use of boron nitride nanotube and nanosheets was also proposed to be a strong CO 2 adsorbent with high selectivity and reversibility. The materials simply adsorb CO 2 by introducing electrons to the adsorbent and then release CO 2 by removing the electrons, which make them strong candidates to be used in DAC system. 111 Chemisorption. Chemisorption materials include both aqueous solutions and amine-modied sorbents. The aqueous solutions were discussed in the previous sections and their main drawbacks are the high regeneration energy due to their high heat capacities, and the heat required for evaporation. On the other hand, amine-modied sorbents offer a strong chemical bond and low regeneration energy, in which amines are infused into the pores of the solid materials. 110 The adsorption of CO 2 using amines groups occurs in two different mechanisms based on whether the condition is dry or wet. The primary and secondary amines adsorb CO 2 to produce carbamate acid with strong bond and carbamic with weak bond, respectively in dry conditions. 113 Chemisorption materials include both aqueous solutions and amine-modied sorbents. The aqueous solutions were discussed in the previous sections and their main drawbacks are the high regeneration energy due to their high heat capacities, and the heat required for evaporation. On the other hand, amine-modied sorbents offer a strong chemical bond and low regeneration energy, in which amines are infused into the pores of the solid materials. 110 The adsorption of CO 2 using amines groups occurs in two different mechanisms based on whether the condition is dry or wet. The primary and secondary amines adsorb CO 2 to produce carbamate acid with strong bond and carbamic with weak bond, respectively in dry conditions. 113,114 The second mechanism in wet condition, the reaction between amines and carbon dioxide forms bicarbonate. 115 The reaction between tertiary amines and carbon dioxide to produce carbamate does not occur because there should be a hydrogen atom that can be replaced by COO − . The reactions between secondary amines and CO 2 for both dry and wet conditions are shown, respectively. 115 The reaction between tertiary amines and carbon dioxide to produce carbamate does not occur because there should be a hydrogen atom that can be replaced by COO − . The reactions between secondary amines and CO 2 for both dry and wet conditions are shown, respectively.
There are different amine-modied sorbents, which can be divided into three preparation-based categories such as category 1, category 2 and category 3 as shown in ref 57 . All the sorbents that involve physical impregnation of amines into porous materials is included in category 1. [116][117][118][119] Category 2 depends on stabilizing the sorbent by chemically graing the amine to the surface of the sorbent. 120 In category 3, the prepared amine containing monomers through in situ polymerization is chemically graed to a sorbent with inorganic  supports. One of DAC's startup companies, Global Thermostat, has patented different amine-modied sorbent for its operation. 78 Category 1. The rst category includes impregnating amines on solid supports to selectively capture CO 2 . There are different types of amines and solid supports reported in the literature as shown in Fig. 8. Low molecular weight and low volatility amines are preferred as this increases the capacity and stability of the sorbents. Small amines, which are used in solution separation such as monoethanolamine are not considered because of their low boiling point. 121 Other linear amines such as pentaethylenehexamine (PEHA) and tetraethylenepentamine (TEPA) have been proposed but they have leaching and amine loss issues. The primary amine used by many researchers is branched poly(ethylenimine) (PEI), which is characterized by high density and stability under TSA and VSA. In general, PEIs are amines that are connected to ethylene, but have different shapes based on the type of amines.
The linear shape is presented when ethylene is connected only to secondary amine while it is branched like trees when tertiary amine is included as illustrated in Fig. 9. 122 The high heat of adsorption was expected to be an indication of higher CO 2 capacity. The primary amines are reported to have higher heat of adsorption compared to secondary amines. The use of poly(allylamine) (PPA) that contains only primary amines has been proposed. However, a comparison between PPA and branched PEI was conducted and the branched PEI achieved higher performance in capturing CO 2 . 123 Later, it was reported that secondary amines give the best balance between CO 2 uptake and energy requirements. 124 Another attempt to improve the efficiencies of amine was the addition of poly(ethylene glycol) (PEG) to PEI. The addition of PEG1000 to PEI-SBA-15 increased the CO 2 uptake from 0.61 to 0.79 mmoleCO 2 per g. 125 The use of hexamethyldisilazane (HMDS) is another way to improve CO 2 capacity of the sorbent. The small-molecule poly(propylenimine) (PPI) and PEI/PVA 126 were proposed to be used instead of PEI to avoid oxidative degradation at high temperatures. Using PPI, the stability was simulated to last over 50 cycles without loss in performance. 127 The internal diffusion of CO 2 inside the PEI lm can be enhanced using diffusion additives such as span 80 (sorbitan monooleate produced by Sinopharm) leading to higher CO 2 capacity. 76 The main solid sorbents associated with the rst category are discussed below. 122 The high heat of adsorption was expected to be an indication of higher CO 2 capacity. The primary amines are reported to have higher heat of adsorption compared to secondary amines. The use of poly(allylamine) (PPA) that contains only primary amines has been proposed. However, a comparison between PPA and branched PEI was conducted and the branched PEI achieved higher performance in capturing CO 2 . Later, it was reported that secondary amines give the best balance between CO 2 uptake and energy requirements. 124 Another attempt to improve the efficiencies of amine was the addition of poly(ethylene glycol) (PEG) to PEI. The addition of PEG1000 to PEI-SBA-15 increased the CO 2 uptake from 0.61 to 0.79 mmoleCO 2 per g. 125 The use of hexamethyldisilazane (HMDS) is another way to improve CO 2 capacity of the sorbent. 128 The small-molecule poly(propylenimine) (PPI) and PEI/PVA 126 were proposed to be used instead of PEI to avoid oxidative degradation at high temperatures. Using PPI, the stability was simulated to last over 50 cycles without loss in performance. 127 The internal diffusion of CO 2 inside the PEI lm can be enhanced using diffusion additives such as span 80 (sorbitan monooleate produced by Sinopharm) leading to higher CO 2 capacity. 76 The main solid sorbents associated with the rst category are discussed below.
Silica support. Mesoporous silica support was rst used by Xu et al. by adding polyethyleneimines (PEI) into the silica pores. This was the rst molecular sieve with the name Mobil Composition of Matter (MCM-41). The structure of the sorbent has effect on how efficient CO 2 binding to amines is, moreover, the structure is temperature-dependent as shown in Fig. 10. 78 The method used to prepare the PEI in (MCM-41) is wet impregnation, where the silica porous material is added to dissolved amines in organic solvent (methanol). The main problem with the mentioned category is the instability of the polymeric amines in humid conditions due to water  condensation, solubilization and their weak interaction to the solid support. 119,129,130 The method used to prepare the PEI in (MCM-41) is wet impregnation, where the silica porous material is added to dissolved amines in organic solvent (methanol). 131 The main problem with the mentioned category is the instability of the polymeric amines in humid conditions due to water condensation, solubilization and their weak interaction to the solid support. 119,129,130 Moreover, the high volatility and low boiling point of monoethanolamine lead to stability issues even in dry conditions. 119 The volatility can be avoided using large molecular weight amines, however, large molecular weight amines have been reported to have negative effects on CO 2 capacity. 118,119,132 The volatility can be avoided using large molecular weight amines, however, large molecular weight amines have been reported to have negative effects on CO 2 capacity. 118,132 Researchers have investigated the use of various PEI with different molecular weights (M W ) between 400 and 25 000 in porous silica to maximize the stability. 129 Amine modied alumina. Alumina 133 and titania 134 were proposed to replace silica to avoid silica's shortcomings. The amine modied silica regeneration methods, such as temperature-vacuum alternation, exposure to water vapor (under 110°C) and hot CO 2 ow lead to instability and collapse of their CO 2 capacity. The silica support stability is limited to below 135°C in dry condition due to urea formation, while SBA-15, MCF and MCM-41 were reported to have low stability in humid conditions. 135 Alumina support crystallinity nature offers a high resistance to degradation when it is regenerated with water vapor. PEI modied mesoporous g-alumina (SynA) regeneration using steam was compared to SBA-15 under the same condition. The CO 2 capacity of SynA was reduced by 25.2% while the reduction was 81.3% in the case of SBA-15. 133 Other amine modied solid supports such as resin, 87 carbon nanotube 87 and mesoporous carbon are shown in Table 3.
DAC relevancy summary. Adsorption of CO 2 in a dry stream using polyethyleneimine modied silica was comprehensively tested under different conditions, such as different adsorptiondesorption cycle temperatures (30-120°C), loading (10-55%), molecular weights (600-2500 Da) to evaluate its cyclic stability, thermal stability, adsorption capacity and kinetics. The results showed that the higher the loading of polyethyleneimine into the silica the better its cyclic stability and CO 2 uptake, but increasing the PEI molecular weight decreases the CO 2 uptake with no noticeable effect on cyclic stability. PEI was found to be thermally stable below 210°C temperature, at higher temperature, PEI starts to evaporate and decompose. The temperature variation tests demonstrate different CO 2 uptake for different PEI loading while the cyclic stability was better at higher temperatures. In general, high loading PEI modied silica show higher CO 2 uptake for higher temperature while for low loading, low temperatures below 60°C is preferred. From DAC point of view, 30-40% loaded PEI modied silica is suitable as it has relatively high capacity in ambient temperature. 136 New PEI modied silica with stabilizing additives, 3-aminopropyltrimethoxysilane and tetraethyl orthotitanate, were developed to adsorb 400 ppm of CO 2 from inert gas stream and the results showed higher thermal stability compared to the conventional PEI; a good cyclic stability in dry conditions and a high CO 2 capacity up to 2 molCO 2 per kg of sorbent. 129 The PEI was integrated into other different silica supports such as  fumed silica, SBA-15, and Zr-SBA-15 as shown in Table 3. The fumed silica (FS-PEI) shows superior performance in DAC system in dry and humid conditions. Its main advantage is that it has high capacity and kinetics in humid conditions. 121 (FS-PEI) cyclic stability was reduced by 4% aer the fourth cycle, so PEI-H + PEG in dry adsorption conditions was used instead of PEI and showed a stability for 20 cycles; however, its CO 2 capacity was lower. 124 The use of SBA as a solid support was proposed also to be used in DAC system and it shows constant stability over 20 cycles and can reach 50 cycles 137 using PPI instead of PEIs, but the CO 2 capacity still was low. 124,125,138 The fumed silica (FS-PEI) shows superior performance in DAC system in dry and humid conditions. Its main advantage is that it has high capacity and kinetics in humid conditions. 121 (FS-PEI) cyclic stability was reduced by 4% aer the fourth cycle, so PEI-H + PEG in dry adsorption conditions was used instead of PEI and showed a stability for 20 cycles; however, its CO 2 capacity was lower. 124 The use of SBA as a solid support was proposed also to be used in DAC system and it shows constant stability over 20 cycles and can reach 50 cycles 137 using PPI instead of PEIs, but the CO 2 capacity still was low. 124,125,138 However, the amine-modied silica shows high capacity and stability under dry condition. The DAC system requires high stability sorbent in humid conditions. Alumina was proposed to capture CO 2 from ambient air as it has high resistance to structural changes in humid conditions. 133,139 Finally, the use of the rst category for developing indoor DAC system can expose humans to the danger of inhaling amines due to the support's oxidative degradation, thermal and cyclic instability or amine group's leaching and high volatility. The main DAC studies including the use of resin and mesoporous carbon are shown in Table 3. Category 2. During regeneration, a more stable category is the second category where different amine groups are chemically graed onto the solid supports. The rst category of chemisorption has a higher CO 2 capacity than the second, however, the rst category suffers from low stability due to the weak physical forces between the amine groups and the supports. 78 In the second category, the amines covalently bind to the solid supports through the active sites (such as silyl, hydroxyl, and alkyl) on their surface. 16 The indicated category can be prepared by two methods, which are silane chemical reactions and binding to coupling agents. The silane chemical reaction involves hydrolyzation or alcoholization of siloxane silane amines groups, which bind to a hydroxy group and condense on the surface of a support, and then a single amine layer is produced. 78 Different studies on the second category will be discussed below and arrange based on the solid supports used.
Silica and its derivatives support. The use of TRI-PE-MCM-41 was rst proposed by Belmabkhout et al. 145 to capture CO 2 from air in humid and dry conditions. TRI-PE-MCM-41 successfully captured CO 2 from a mixture containing N 2 , O 2 , H 2 O vapor, and CO 2 and showed high selectivity towards CO 2 . It demonstrates higher performance in CO 2 capture compared to physisorption materials like Zeolites and MOFs. 145 The effect of the support structure, such as specic surface area, pore size, and pore volume, was evaluated. The studies showed that larger pore sizes and pore volume of the support (SBA-15) offer better adsorptivity properties because they allow higher amine density. The effect of steam treatment on amine-graed silica was also investigated, and the results show that CO 2 capacity is reduced especially at low temperatures where the reduction becomes signicant. It was illustrated that commercial-grade silica (P10) has better hydrothermal stability when compared to SBA-15. 146 Alumina supports. The resilience of alumina to steam treatment is high compared to silica, which makes alumina more stable. One more difference between alumina and silica is that alumina can be tuned to be either acidic or basic. 3-Aminopropyltriethoxysilane (APS) was graed onto two alumina support types, and it was found that different amine species are formed once the alumina interacts with carbon dioxide. The reported CO 2 capacity was between 0.15 and 0.75 mmol (CO 2 )/ g(sorbent), which is lower than the reported capacity for amine graed silica supports when it was adsorbed from N 2 stream with CO 2 concentration of 500 ppm. 147 Nano brillated cellulose (NFC) & porous polymer networks (PPN). The Nano brillated cellulose consists of cellulose brils aggregates that exist naturally and are rich in hydroxyl groups. 148 The use of NFC shows high stability and high CO 2 capacity in direct air capture of CO 2 . 79 The decrease in CO 2 capacity of NFC aer 100 cycles was only 5%, and its capacity ranged from 39.6 to 93.7 mg of CO 2 per gram of sorbent as illustrated in Table 4. Researchers have also investigated the use of porous polymer networks (PPN) as a support instead of silica. PPNs offer higher amine loading and CO 2 capacity compared to silica because of their three-dimensional structure and high porosity. A comparison was held between (PPN-6-CH 2 DETA) and TRI-PCM-40 (silica support) at ambient conditions. The CO 2 capacity of PPN-6-CH 2 DETA (43.1 mg g −1 of sorbent) was higher by 6%, and the heat of sorption was lower by 25%. 31 MOF supports. Ethylenediamine based metal-organic frameworks (Mg/dobpdc) was used to capture CO 2 from ambient air, and its CO 2 capacity was 125.4 mg of CO 2 per gram of adsorbent, 7 which is higher than other MOFs adsorbents. 149 The isotherm shape and kinetics of Mg/dobpdc for CO 2 adsorption under dilute air conditions lead to lower CO 2 adsorption. It was found that the mentioned adsorbent did not fully saturate with CO 2 under the low CO 2 partial pressure in the atmosphere due to the strong molecules' bonds between the two adjacent amine groups. The authors raised the adsorbent CO 2 capacity to 171.19 mg g −1 under atmospheric conditions by using diamine: hydrazine (H 2 N 4 ) instead of ethylenediamine. 150 Brønsted acid-base reactions method was used to tether alkylamines to the Cr-MIL-101-SO 3 HMOF. The developed adsorbent under the optimal conditions showed a CO 2 capacity of 49.29 mg g −1 CO 2 from the ambient air. 151 DAC relevancy summary. The major issue with using the rst category in DAC systems is that the material losses stability, which implies a higher cost besides the environmental concern of releasing the adsorbent material directly into the atmosphere. 78 The second category was proposed to overcome these issues, however, a reduction of CO 2 capacity was noticed. A comparison between different primary, secondary and tertiary amines graed on mesoporous silica was carried out to evaluate which one is the most suited to ambient air conditions. It was found that amines with higher primary amines have higher CO 2 capacities and stronger water affinity. Moreover, it was reported that lower adsorption temperature achieved higher CO 2 capacity, which is relevant for DAC systems. 152 Diaminefunctionalized silica gel was used to capture CO 2 from ambient air using the TVS method, and it shows cyclic stability up to 40 cycles, but its CO 2 capacity was very low compared to other adsorbents. 33 The amine-based nano brillated cellulose was also proposed to be used in DAC, and it showed a high cyclic stability as it decayed only 5% aer 100 cycles under humid conditions, but the presence of O 2 led to sorbent degradation. 148 Although the adsorption of H 2 O with CO 2 using APS based NFC was reported to increase the CO 2 capacity, it also contributed to higher energy requirements at the regeneration stage. 79 The limitation raised from adsorption of water, such as high regeneration energy and instability at high temperatures, motivated the integration between hydrophobic aryl moieties and alkylamines to increase the selectivity of alkylamines to CO 2 over water. Different amines bearing benzene moieties were investigated, and among them, m-xylylenediamines(MXDA, 4e), o-xylylenediamines(OXDA, 4g) and p-xylylenediamines(PXDA, 4f) did not adsorb any water with the CO 2 . 80 Although the second category offers a strong chemical bond between amines and the supports, the solid supports self-degraded aer a number of cycles and volatilize with amines. The mentioned issue discourages the use of second-category materials in indoor units. The main studies parameters associated with chemisorption class 2 are shown in Table 4.
Category 3. Although class 2 adsorbents were reported to be stable over 100 cycles in humid conditions, 31,157 other studies showed that the stability is reduced below category 1 level upon exposure to water. 158 Moreover, the second category adsorbents have low CO 2 capacity due to the low loading of amines compared to other categories. The third category involves covalently bound polymeric amines on a porous solid support, which allows for higher amine loading. These types of sorbents are called hyper-branched amino silica (HAS), and they were developed by Choi et al. 158 These sorbents offer high capacity, stability, easy preparation, low cost, and excellent regeneration compared to category 2. It was reported that the CO 2 capacity of these adsorbents increases linearly, from 8.8 to 66 mg g −1 of adsorbent, with more amine loading under dilute air conditions. 159 Other examples of category 3 are functionalized SBA-15 by melamine-based dendrimers and hyper-branched amino silicas. 158 Moreover, the second category adsorbents have low CO 2 capacity due to the low loading of amines compared to other categories. The third category involves covalently bound polymeric amines on a porous solid support, which allows for higher amine loading. These types of sorbents are called hyperbranched amino silica (HAS), and they were developed by Choi et al. 158 These sorbents offer high capacity, stability, easy preparation, low cost, and excellent regeneration compared to category 2. It was reported that the CO 2 capacity of these adsorbents increases linearly, from 8.8 to 66 mg g −1 of adsorbent, with more amine loading under dilute air conditions. 159 Other examples of category 3 are functionalized SBA-15 by melamine-based dendrimers and hyper-branched amino silicas. Poly(L-lysine) brush-mesoporous silica hybrid material was also used to capture CO 2 from dilute air and it was reported to be stable over three cycles with CO 2 capacity of 26.4 mg of CO 2 per gram of adsorbent under ambient air conditions. 160 Chemisorption regeneration methods. One more important parameter of CO 2 capture beside adsorbent CO 2 capacity is the regeneration. The allowable cost of the whole CO 2 capture and regeneration cycle will dramatically decrease if the capacity is low which ultimately leads to unrealistic process from the economy point of view. 161 One of the regeneration methods is the pressure swing adsorption (PSA), which is viable for post combustion capture, 162 but PSA requires very high compression or unpractical vacuum level for it to be applicable to DAC, which makes it impractical for DAC application. 153,163 The most oen-used regeneration method for amine-based adsorbent in the laboratory scale is temperature swing adsorption. 164 Although the TSA needs simple design, most amines degrade if desorption temperature reaches above 100°C, moreover, it produces dilute CO 2 stream. 165 The desorption of CO 2 using inert gas instead of air can successfully solve the issue of oxidative degradation; however, it increases the process cost, which is not practical for DAC systems in which cost of capture is already high. The high CO 2 purity stream could be achieved by TSA using CO 2 as the stripping gas, but the issue is the formation of urea, which deactivates the adsorbent 166,167 One more important parameter of CO 2 capture beside adsorbent CO 2 capacity is the regeneration. The allowable cost of the whole CO 2 capture and regeneration cycle will dramatically decrease if the capacity is low which ultimately leads to unrealistic process from the economy point of view. 161 One of the regeneration methods is the pressure swing adsorption (PSA), which is viable for post combustion capture, but PSA requires very high compression or unpractical vacuum level for it to be applicable to DAC, which makes it impractical for DAC application. 153,163 The most oenused regeneration method for amine-based adsorbent in the laboratory scale is temperature swing adsorption. 164 Although the TSA needs simple design, most amines degrade if desorption temperature reaches above 100°C, moreover, it produces dilute CO 2 stream. 165 The desorption of CO 2 using inert gas instead of air can successfully solve the issue of oxidative degradation; however, it increases the process cost, which is not practical for DAC systems in which cost of capture is already high. The high CO 2 purity stream could be achieved by TSA using CO 2 as the stripping gas, but the issue is the formation of urea, which deactivates the adsorbent 166,167 and reduces the adsorption cycle working capacity. Another way for achieving high purity CO 2 stream by TSA is to use saturated steam as the stripping gas and then condense the water from the product gas, 158,168 the main issue with using steam as the stripping gas is the leaching of amine, which signicantly reduces the cyclic stability 136,140 Another way for achieving high purity CO 2 stream by TSA is to use saturated steam as the stripping gas and then condense the water from the product gas, 158,168 the main issue with using steam as the stripping gas is the leaching of amine, which signicantly reduces the cyclic stability. 136,140 The combination of vacuum with TSA is used as a regeneration method called temperature-vacuum swing adsorption (TVSA), it reduces the desorption temperature to below 100°C; however, amine leaching is still the associated drawback when steam is used as the stripping gas. 169 TVSA is capable of producing almost 100% pure CO 2 , but the required temperature swing will be high, and the working capacity will reduce when compared to TSA and temperature concentration swing adsorption (TCSA) 153,163 used as the stripping gas. 169 TVSA is capable of producing almost 100% pure CO 2 , but the required temperature swing will be high, and the working capacity will reduce when compared to TSA and temperature concentration swing adsorption (TCSA). 153,163 The reason for the mentioned changes is that TVSA required closed inlet during desorption to prevent dilution of the product gas, which means that desorption occur in high CO 2 concentration chamber. 170 A comparison of CO 2 capacity of diamine-functionalized silica gel based on regeneration methods (TCSA and TVSA) used. 163 It was found that TCSA and TVSA at desorption temperature of 90°C and vacuum pressure of 50 mbar achieved CO 2 capacity of 19.36 and 11.88 mg of CO 2 per gram of sorbent, respectively. It was reported that the increase in desorption temperature could increase the working capacity, but it is limited by thermal degradation. Moreover, the working capacity of adsorbent using TVSA can be increased by the regeneration when purge gas such as inert gas or air is used. However, this is not proposed for DAC systems due to the large energy penalty required for its implementation, moreover, it does not produce pure CO 2 stream. In the literature, a comparison between desorption rate using steam or inert gas as the stripping gas with TVSA has been reported. 169 The adsorbent regenerability is as important as working capacity and specic energy requirement (SER), as it signicantly affects the DAC system cost, 161 20. 169,171 The adsorbent regenerability is as important as working capacity and specic energy requirement (SER), as it signicantly affects the DAC system cost, 161 however, most amine-modied stability experiments are done through few cycle numbers below 20. 167 Most of the DAC system  177 The required CO 2 separation power has been estimated to vary from 1250 kW h t −1 to (3471-4418) kW h t −1 for CO 2 production with purity less than 5% to more than 90%, respectively. 178 The previous estimate shows that it is highly important to choose different regeneration methods based on the targeted CO 2 purity. There are other applications which do not need 100% pure CO 2 such as microalgae cultivation, 174 greenhouses 175,176 and microbial cultivation. 177 The required CO 2 separation power has been estimated to vary from 1250 kW h t −1 to (3471-4418) kW h t −1 for CO 2 production with purity less than 5% to more than 90%, respectively. 178 The previous estimate shows that it is highly important to choose different regeneration methods based on the targeted CO 2 purity. Different regeneration methods associated with low CO 2 purity product such as TSA, TCSA and TVSA with air or inert gas purge ow were compared to closed TVSA without purge ow, which yields nearly 100% pure CO 2 stream. The mentioned methods were compared based on adsorbent regenerability, working capacity (WC) and specic energy requirement (SER) for proprietary amino resin on silica support as shown in Table 5.
In the study, two desorption temperatures were used, which are 60°C and 100°C representing 90% and 99% of the maximum working capacity, respectively. All the experiments were carried out under dry conditions. The results demonstrated that inert gas or air based 500 mbar TVSA records higher working capacity than TSA, TCSA and closed TVSA; however, the least SER was achieved by isobaric TSA and TCSA. As a conclusion, the purge gas integrated to TVSA with mild vacuum pressure offers high working capacity and reasonable specic energy requirements, which makes it more viable for low CO 2 concentration applications. 179 Moisture swing based chemisorption. Moisture swing mechanism. Through the moisture swing, the adsorbents bind to CO 2 under dry condition and then the CO 2 is released when the adsorbents are exposed to moisture. The moisture swing technology was initially proposed to be used for CO 2 capture from ambient air by Lackner. 180 Most of the adsorbents that use moisture swing method are quaternary ammonium ions integrated with resin, which is considered a strong basic ion exchange resin. The role of quaternary ammonium ions is similar to the role of sodium cation in aqueous solution. In carbon capture application using moisture swing, the integrated cation is balanced by hydroxide, carbonate and bicarbonate anions, which their abundance depends on the CO 2 and moisture loading. 181 The moisture swing mechanism can be explained based on four main steps as shown in Fig. 11. In the rst step, the carbonate ions are attracted to the positive quaternary ammonium ions in the presence of water. The existence of excess water can stabilize carbonate, hydroxide or bicarbonate ions but since the carbonate hydration is stronger than the others, only the carbonate exist in the rst step. In the second step, the adsorbent is dried to reduce the water content, which leads to less stable carbonate ions. The carbonate ions tend to split and react with the remaining water molecules, forming bicarbonate ions. Since the water content is reduced, there will be hydroxide ions that has high binding affinity to carbon dioxide. The reaction between both hydroxide ions and carbon dioxide leads to adsorption of CO 2 and forming bicarbonate as in the third step. The reactions involved in the second and third steps are as follow: Moving from the third step to the fourth requires the exposure of the adsorbent to moisture, which increases the presence of bicarbonates over the existed carbonate and carbon dioxide, leading to a reaction (19) where the bicarbonates are split into carbonate ions, carbon dioxide and water. The carbon dioxide is being desorbed in this step while the presence of water and carbonate ions results in the cycle repetition. Based on the previous fourth step, the adsorption and desorption of carbon dioxide can be controlled by controlling the amount of water present. 182 DAC relevancy summary. The use of ion exchange resin (IER) in DAC system was proposed since the energy required for traditional thermal swing or pressure swing adsorptions can almost be eliminated. Instead, the air moisture content changes can be used to implement the complete adsorption and desorption cycle. Most traditional DAC systems require high estimated energy for regeneration, 179, 89,91 135 and 80 57 kJ per molCO 2 for calcination process, decomposition of Na based sorbents, and amine based respectively, compared with IER which was reported to have low desorption heat of 32 kJ per molCO 2 . Moreover, it was experimentally reported that by increasing the humidity from 15.8% to a fully humidied condition, the equilibrium partial pressure of CO 2 in IER increases by two orders of magnitude. 183 Although the moisture swing needs relatively clean water to avoid resin contamination, and it produces CO 2 stream with low purity, the vapor in air can be used to avoid water cleaning energy penalty, and the purity could be increased by integrating thermal swing or vacuum assisted mode to the technology. The low partial pressure of released CO 2 stream was increased using improved binder that increased the air/sorbent contact area. 184 The slow kinetics and low capacities of IER are considered as major disadvantages of the technology. Hou et al. 185 tried to overcome this issue by preparing a moisture swing based polymeric material and the kinetics was signicantly improved compared to other similar functionalized groups. Capturing CO 2 from air using IER dispersed in polypropylene through moisture swing was investigated to determine the isothermal performance of the adsorbent. 183 The effect of temperature and particle size on CO 2 loading half time was studied by Wang et al. 184 and the authors showed that half time decreases from 150 to 40 minutes by increasing the adsorption temperature from 0 to 30°C. The half time of sorbents uses moisture swing technology was a major concern, which motivated researchers [186][187][188] to develop new sorbents with improved sorption rates under ambient conditions. The moisture swing technique was demonstrated to be applicable with sorbents other than resin. The nanomaterials show formation of carbonate ions, which can split water into bicarbonate and hydroxide by controlling the amount of moisture. 182 One example of these nanomaterials is Na 2 CO 3 impregnated in activated carbon powder. 189 A numerical and experimental investigation of moisture swing mechanism and the effect of surface hydrophobicity, pore size, and spacing of cations on CO 2 capture efficiency was implemented. 181 In the indicated study, ve samples were tested but only two samples show sensitivity to moisture level as shown in, 89,91 135 and 80 kJ per molCO 2 for calcination process, decomposition of Na based sorbents, and amine based respectively, compared with IER which was reported to have low desorption heat of 32 kJ per molCO 2 . 76 Moreover, it was experimentally reported that by increasing the humidity from 15.8% to a fully humidied condition, the equilibrium partial pressure of CO 2 in IER increases by two orders of magnitude. 183 Although the moisture swing needs relatively clean water to avoid resin contamination, and it produces CO 2 stream with low purity, the vapor in air can be used to avoid water cleaning energy penalty, and the purity could be increased by integrating thermal swing or vacuum assisted mode to the technology. 78 The low partial pressure of released CO 2 stream was increased using improved binder that increased the air/sorbent contact area. 184 The slow kinetics and low capacities of IER are considered as major disadvantages of the technology. Hou et al. 185 tried to overcome this issue by preparing a moisture swing based polymeric material and the kinetics was signicantly improved compared to other similar functionalized groups. Capturing CO 2 from air using IER dispersed in polypropylene through moisture swing was investigated to determine the isothermal performance of the adsorbent. 183 The effect of temperature and particle size on CO 2 loading half time was studied by Wang et al. 184 and the authors showed that half time decreases from 150 to 40 minutes by increasing the adsorption temperature from 0 to 30°C. The half time of sorbents uses moisture swing technology was a major concern, which motivated researchers 186,187 to develop new sorbents with improved sorption rates under ambient conditions. The moisture swing technique was demonstrated to be applicable with sorbents other than resin. The nanomaterials show formation of carbonate ions, which can split water into bicarbonate and hydroxide by controlling the amount of moisture. 182 One example of these nanomaterials is Na 2 CO 3 impregnated in activated carbon powder. 189 A numerical and experimental investigation of moisture swing mechanism and the effect of surface hydrophobicity, pore size, and spacing of cations on CO 2 capture efficiency was implemented. 181 In the indicated study, ve samples were tested but only two samples show sensitivity to moisture level as shown in Table 6. Other Different investigated moisture swing-based DAC systems are shown in Table 6. Although the moisture swing DAC systems saves large amount of energy, it has low kinetics and low capacity compared to other methods.
Other CO 2 capture classications Nanomaterials for DAC. An efficient material for DAC systems should have a high adsorption capacity, low regeneration energy and high stability. These properties can be achieved using nanomaterials as they have chemical and thermal stability, high surface area, and accessible pore regions. Nanomaterial can be classied into three main categories: zerodimensional nano-objects, one-dimension nano-objects, and nanostructured materials. Nanoparticles and nanouids are included in the rst category. Nanoparticles are small particles; their sizes vary from 1-100 nm and can be fabricated from different metals or core materials. 192 Nanouids are uids with suspended nanoparticles, and their three-phase nature reduces the energy demand and enhances the absorption capacity of CO 2 by increasing the mass transfer coefficient. 193 Nanober and nanotubes 145 are grouped under the one-dimension nanoobjects. Nanobers are nano-dimensional materials, but their length is larger than nanoscale. Nanobers are fabricated from polymers such as cellulose nanobers 194 and metal nano-bers. 195 Finally, nanostructured materials include nanocrystalline, nanocomposites, and nonporous materials. Nanocrystalline can be multi or single polycrystalline solids that have sizes less than 100 nm, such as functionalized BN nanosheets, 196 cellulose nanocrystals, 189 and metal dioxide nanocrystalline. 197 The nanocomposites are a combination of different nano-objects with sizes less than 100 nm, and they can be in one, two, or three dimensions. 198 The last in the category of nanostructured materials are nonporous materials with a framework having pore sizes less than 100 nm. 199 In Table 7, many nano-based CO 2 capture systems show high capacity and stability over a few cycles; however, all the experiments were not carried out at atmospheric CO 2 concentration or in the presence of moisture. Including the ultra-dilute concentration levels and moisture contents can dramatically decrease the capacity and stability. Poly HIPE/nano-TiO 2 /PEI-50 was tested in the presence of both water and nitrogen and showed a high capacity and stability, but the tested concentration levels were not to the same as the atmospheric one, which makes poly HIPE/nano-TiO 2 /PEI-50 a potential sorbent that needs further investigations. 200 CNF-Ph(1 : 1.5) was equally investigated, and the results showed a high capacity under DAC conditions; moreover, it was indicated that the increase in humidity further increased the CO 2 capacity. 75 Yanhao Deng et al. 75 reviewed the performance of nanomaterials in DAC conditions, and it was concluded that the best performance achieved by cellulose nanobrils under low concentrations, but suffered from lack of stability. On the other hand, nanosheets and nanostructured materials showed high stability, although they are not effective in capturing CO 2 from ambient air. 201 Green sorbents in DAC. The concept of removing CO 2 from the atmosphere attracted many research interests around the world, but the difficulty and the general lack of applicability involved are the challenges in DAC. 203,204 Different materials were proposed for DAC, such as metal hydroxide-based absorption and amine-supported materials. 78,164 The metal hydroxide materials are lightweight, small in size, have large capacity, high stability, and good reliability, but they require high regeneration energy. The solid amine-based adsorption materials overcome the high regeneration energy penalty, but they have low CO 2 capacity, and many have instability issues. 78 All the mentioned materials are chemically prepared, implying energy and environmental impacts. As a result, green sorbents were proposed to meet the sustainability requirements. Eliminating environmental threats by using green chemistry attracted many researchers to explore green sorbents since the 12th Global Congress on Process Safety (GCPS). 205 The green sorbents used for CO 2 capture are bioregenerative materials and Ionic liquids (ILs), and they can adsorb CO 2 and release water and oxygen. Many bioregenerative materials were used for CO 2 capture from ue gas stream, such as mesoporous chitosan-SiO 2 nanoparticles (NPs) with a capacity of 193.16 mg of CO 2 per gram of sorbent 205 and superhydrophobic PVDF/Si-R hollow ber membrane, which shows a better performance than MEA. 206 Eliminating environmental threats by using green chemistry attracted many researchers to explore green sorbents since the 12th Global Congress on Process Safety (GCPS). 205 The green sorbents used for CO 2 capture are bioregenerative materials and Ionic liquids (ILs), and they can adsorb CO 2 and release water and oxygen. Many bioregenerative materials were used for CO 2 capture from ue gas stream, such as mesoporous chitosan-SiO 2 nanoparticles (NPs) with a capacity of 193.16 mg of CO 2 per gram of sorbent 205 and superhydrophobic PVDF/Si-R hollow ber membrane, which shows a better performance than MEA. 206 The outstanding characteristics of ILs make them an effective material for CO 2 capture. Many studies have been conducted to investigate the physical properties of ILs. 206 The use of ILs in carbon capture can be classied into three categories, which are amino acid ionic liquids, 207 ILs-cosolvent, 202 and ILs-supported materials. 206 The amino acid ionic liquids (AAILs) are green sorbents that can replace the aqueous amine solution, and they show a higher adsorption rate compared to pure amino acids, amino acid salts, or the derivatives of ethanolamine. 207 Different cations were integrated with AAILs such as chlorine, imidazole cations, phosphonium, and amines to be used in carbon capture. [208][209][210] The  211 Although these materials demonstrated high CO 2 capacity and were shown to be stable, experiments under real atmospheric conditions and in the presence of moisture are required. There are two main problems associated with the application of AAIL in CO 2 capture: the limited capacity under low CO 2 concentration and high viscosity. Different ammonium based AAILs successfully achieved lower viscosity. 205,212 The amino acid ionic liquids (AAILs) are green sorbents that can replace the aqueous amine solution, and they show a higher adsorption rate compared to pure amino acids, amino acid salts, or the derivatives of ethanolamine. 207 Different cations were integrated with AAILs such as chlorine, imidazole cations, phosphonium, and amines to be used in carbon capture. 208 211 Although these materials demonstrated high CO 2 capacity and were shown to be stable, experiments under real atmospheric conditions and in the presence of moisture are required. There are two main problems associated with the application of AAIL in CO 2 capture: the limited capacity under low CO 2 concentration and high viscosity. Different ammonium based AAILs successfully achieved lower viscosity, 205,212 and the use of choline cation was reported to be an effective green sorbent in capturing CO 2 from direct air. 213 The main disadvantage of using choline-based AAILs is their decomposition under heating and their strong water adsorption.
The use of co-solvents such as polymer, water, alcohol, and eutectic solvent with pure ILs was proposed to decrease its high viscosity. 214 The integration of polyethylene glycol (PEG) with ILs was investigated in different CO 2 capture studies because of its low viscosity and cost compared to other co-solvents. 215 The use of co-solvents such as polymer, water, alcohol, and eutectic solvent with pure ILs was proposed to decrease its high viscosity. 214 The integration of polyethylene glycol (PEG) with ILs was investigated in different CO 2 capture studies because of its low viscosity and cost compared to other co-solvents. 215 However, adding the co-solvent to ionic liquid was able to decrease its viscosity, the rate of CO 2 mass transfer was nevertheless still low. Integrating ILs on solid supports increased the adsorption rate compared to pure or co-solvent-based ILs. Many studies investigated the use of ILs on different solid sorbents such as porous silica, 216 TiO 2 surface, 217 porous microsphere PMMA, 218 supported ILs-membranes, 215 and ILs capsules. 219 Most of the experiments associated with the use of ILs on solid supports were conducted under ue gas stream with conditions different from those of DAC. However, the ILs based capsules were used to capture CO 2 from a 100% humid stream with 5000 ppm of CO 2 /N 2 . The results showed high selectivity over N 2 , CO 2 capacity of 66 mg CO 2 /g of sorbent and good stability, 217,220 porous microsphere PMMA, 218 supported ILsmembranes, 215 and ILs capsules. 219 Most of the experiments associated with the use of ILs on solid supports were conducted under ue gas stream with conditions different from those of DAC. However, the ILs based capsules were used to capture CO 2 from a 100% humid stream with 5000 ppm of CO 2 /N 2 . The results showed high selectivity over N 2 , CO 2 capacity of 66 mg CO 2 /g of sorbent and good stability. 220 In summary, carbon capture eld requires the use of green and sustainable sorbents. Different types of ILs show good performance in CO 2 capture, but there are no DAC-based ILs studies in the literature to show how efficient ILs are in removing CO 2 from dilute air. A general limitation of ILs is the complex and costly preparation methods. 204 Other CO 2 sorbents Supported Alkali carbonates. The replacement of hydroxide solutions by inorganic solids was studied by Steinfeld and coworkers 100 as solid supports can increase the active alkali surface area, which improves the carbonation rates. In these sorbents, the alkali carbonates are impregnated into solid porous material. A prepared porous carbon support was loaded with MgO and CaO to capture CO 2 and the study 219 showed its adsorption increase with increase in moisture contents. The CO 2 capture performance of potassium carbonate on different supports was conducted, and K 2 CO 3 /Al 2 O 3 showed the highest CO 2 capacity. Although K 2 CO 3 /Al 2 O 3 has the highest capacity, it needs a high regeneration temperature of 350°C, while K 2 CO 3 / AC and K 2 CO 3 /SG can be generated in temperatures between 100 and 200°C. 221 Supported Alkali carbonates DAC studies characteristics are shown in Table 8.
Nanostructured carbon nitrides. Carbon nitride (CN) is a material linked to nitrogen by covalent bonds in a 2-D shape. The CNs are favorable material in capturing CO 2 because of the existence of nitrogen on its surface, stability, and being green sorbents. Although carbon nitride has a nonporous nature which reduces its CO 2 capturing capacity, it can be integrated with another porous support to improve its performance. At 23°C and 0.93 bar, the CO 2 capacity of melamine-based g-C3N4 nanosheets was as low as 7.9 mg g −1 of sorbent. The capacity was improved to 33.4 and 55.5 mg g −1 by incorporating BIF-20 and ZIF-8, respectively. 226 The adsorption performance of PEIbased g-C3N4 was investigated, and a capacity of 75 and 165 mg g −1 was achieved at temperature of 25°C and 100°C, respectively. 227 Moreover, the electronic proprieties of carbon nitride can bind and release CO 2 by change in voltage and this method shows high selectivity to CO 2 compared to N 2 , H 2 , and CH 4 . However, the CO 2 capacity achieved using the previous approach is low (4.2 mg g −1 ), there are no energy barriers in binding and releasing CO 2 . 228 Different mesoporous carbon nitride (MCN-7-100, MCN-7-130, and MCN-7-150) performance in CO 2 adsorption was tested, and MCN-7-130 achieved a superior capacity of 594 mg g −1 . The reported capacity was under conditions of 0°C and 30 bar, while when the CO 2 pressure decreased to the atmospheric pressure, the capacity dropped to 61.6 mg g −1 . 229 In summary, the carbon nitride sorbents experiment in the literature is limited to CO 2 capture from ue gas, and further DAC studies are required, especially for the approach that uses only voltage change in binding and releasing CO 2 .

Commercialization of DAC
Many companies work on direct air capture as shown in Fig. 12. Keith established Carbon Engineering company in Canada in 2009, in which Bill Gates partly funded it. 69 Carbon Engineering initially used sodium hydroxide solution as a sorbent with a desorption temperature of about 900°C. The company has recently optimized the solvent used from NaOH to KOH to enhance CO 2 capture efficiency of their system. 230 The company introduced a carbon capture unit that can capture 1 ton of CO 2 per day in 2015 with a target to integrate a fuel production system based on the captured CO 2 . The company reported that it could produce 1 barrel of fuel per day using their AIR-TO-FUEL pilot in 2017. The cost of capture, purication, and compression of CO 2 to 150 bar was estimated to be in the range of 75-113 V per tCO 2 . 63 Another top DAC startup in the world today is Climeworks. Gebald and Wurzbacher established Climeworks company in Switzerland in 2009. Climeworks uses amine-modied sorbent to capture CO 2 at a desorption temperature of 100°C. Climeworks collaborated with Audi and Sunre to launch a DAC unit that converted the captured CO 2 into synthetic diesel in 2014. 231 Two other commercial-scale DAC units were installed in Switzerland and Iceland to feed a greenhouse with CO 2 and for the mineralization process, respectively in 2017. The company has a goal to achieve a cost of 75 V per tCO 2 for the large-scale plants. In 2021, the Orca unit was launched in Iceland to capture CO 2 from ambient air and store it through mineralization. The plant runs purely on renewable energy and can capture 4000 tons of CO 2 per year. 232 Eisenberger established Global Thermostat company in the USA in 2010. Global Thermostat uses a low temperature-based system to capture CO 2 from both point sources and ambient air at the desorption temperature between 85 and 95°C. 233 The company launched a running unit that uses amine-modied monolith as a sorbent in California. It was reported that the waste heat with mentioned desorption temperatures could be used in a plant with a capacity of 40 000 tons of CO 2 per year, and they target a capture cost of 11-38 V per tCO 2 . 65 O'Connor established Antecy company in Finland in 2010. It was also working on a low-temperature sorbent that can be regenerated at a temperature between 80-100°C 234 later in 2019. Antecy and Climeworks companies were fully merged to achieve a better and stronger technology. 233 Another DAC system with a CO 2 capacity of 1.387 tCO 2 per year uses the VSA regeneration method with regeneration temperature between 70 and 80°C, the company was established by Oy Hydrocell. 229 Moreover, the company provides laboratory-scale regenerative and nonregenerative CO 2 scrubbers units. 235 There is also Skytree company located in the Netherlands. The information about the technology used by Skytree is limited, but they use electrostatic absorption and moisture for regeneration and benzylamines functionalized sorbent. 236 Besides Skytree, there is also Innitree which was established in New York in 2014, and it launched a laboratory scale unit that uses moisture swing technology in capturing CO 2 . 237,238 All other mentioned companies provide CO 2 purity of more than 99% except companies like Skytree and Innitree, which provide low purity CO 2 , for example, Innitree offers CO 2 purity between 3% and 5% for growing algae. 238 Other newly established companies are creating innovating ideas to make direct air capture more visible such as Aircapture, Carbon Capture, Carbyon, Heirloom, Mission Zero Technologies, and Solitaire Power. Bill Gross established Carbon Capture company in California in the USA in 2019. 239 Carbon Capture company uses molecular sieve technology to remove CO 2 from air with a pre-dehumidier stage powered by low-cost renewable energy. They reported that their system could produce a ton of clean distilled water with every ton of CO 2 capture. 240,241 Carbyon is another DAC company that was established in the Netherlands in 2019. They target a cost-effective system based on amine-modied thin membrane and fast swing technology where there is a rotating drum with a modied material to capture CO 2 effectively. They claimed that the CO 2 adsorption takes 30 seconds and additional 30 seconds is needed for regeneration. They currently target CO 2 capture cost of 50 EUR per ton. 242 Direct air capture using carbon mineralization is developed by Heirloom, a company established in South India. The carbonate minerals are decomposed into pure CO 2 stream and oxide minerals in their system. The oxide mineral is exposed to ambient air to naturally absorb CO 2 without any fans. 241,243 Mission Zero technology was also launched in England in 2020 and uses electrochemical method for sorbent regeneration. 241 Soletair Power was established in Finland in 2016, it uses buildings as carbon sinks by using resin-based DAC technology. The company proclaimed that their products boost human activity and ght climate change. They have two main products: indoor CO 2 capture unit and building HVAC integration unit. Their products can reduce CO 2 concentration in the airow between 200-300 ppm and capable of adsorbing 20 tons of CO 2 per year. However, the building HVAC integration unit is permanently xed to the building HVAC system, whereas the indoor unit is mobile and can be moved depending on the desired location. 241,244

DAC and HVAC integration potential
Buildings consume almost 40% of the world's energy demands, and about 50 to 82% of buildings' energy is consumed in the HVAC systems. 245 The necessity to provide fresh indoor air, especially during and aer COVID-19, 246 and the rising energy demands, challenge researchers to reduce energy losses in the HVAC systems. 247 Minimizing energy demands in buildings can be achieved in two ways: increasing the efficiency of the HVAC system (inside uses) or capturing the lost energy to be used for valuable purposes (outside uses). 247,248 The building energy management systems (BEMS) were used to decrease energy consumption in HVAC systems based on the classications shown in Fig. 13. The inside use means the recovered energy from the HVAC system is used within the HVAC system equipment to raise its efficiency in achieving thermal comfort, while the outside use means the energy is used in another system to provide a useful output such as electricity. 249 Besides the mentioned two classications, DAC with HVAC system integration can be proposed to combine both inside and outside uses. The inside uses include reducing indoor CO 2 levels to improve indoor air quality (IAQ) and HVAC energy reduction using higher air recirculation ratios, while the outside uses include a more efficient DAC system by capturing from more elevated CO 2 concentrated streams (indoor air), applying cooler adsorption and using moisture swing adsorption (MSA) within the HVAC system.

Benets of DAC and HVAC integration
Reducing indoor CO 2 levels is mandatory because indoor CO 2 accumulates due to human metabolism or emissions from indoor sources. 250 The high levels of CO 2 exposure lead to severe cognitive effects. Studies have shown that the CO 2 concentrations in the range of 4000 to 10 000 cause malaise, headache and lethargy, while higher levels of 10 000 to 30 000 may cause metabolic changes, non-narcotic central nervous system and electrolyte imbalances. The indoor CO 2 concentration can be controlled by simply replacing the indoor air with outdoor air through ventilation, however, ventilation is an energy-intensive process that adds more energy requirements for fans, heater, cooler, humidier and dehumidiers. Moreover, in some cases ventilation is not a solution due to the use of air exchange rate reduction strategies or in case of the need of protecting people from outdoor hazards (shelter in place). Gall et al. 251 targeted reducing the CO 2 level in indoor environment such as sleeping microenvironments, school classrooms and shelter in place (SIB) facilities as it can exceed the permissible exposure limit (PEL) of 5000 ppm in 8 hours set by Occupational Safety and Health Administration (OSHA). The study investigated four alkaline metal oxides and hydroxides Ca(OH) 2 , Mg(OH) 2 soda lime and MgO in indoor air cleaning applications. The results showed that mg-containing sorbent has slow kinetics, then soda lime was used, and 1.7 kg was enough to reduce 80% of indoor CO 2 levels and not exceed the OSHA PEL in SIB while it reduced the CO 2 levels from 2599 ppm to 550-750 ppm in low ventilated bedrooms. The simulated pressure drop was 300 pa which can be achieved by typical fans from HEPA lter-containing portable air cleaner. However, CO 2 saturated the sorbent aer 40 hours based on assumed regular occupancy, implying the need for sorbent replacement, leading to high cost. Secondly, the HVAC energy reduction can be achieved by increasing the air recirculation rate. The buildings' energy consumption is high and contributes to one-third of the total global CO 2 emissions. However, the use of CO 2 capture device can reduce the CO 2 levels in building, it does not allow the full recirculation of air because of other pollutants like volatile organic compounds and dust, which are harmful to humans. Most air pollutants other than CO 2 can be captured using lters such as HEPA H14, carbon lter, G3/4 pre-lter and F7/8 ne-lter. Although combining these lters with a CO 2 capture unit removes all types of air pollutants, 252 the build-up of oxygen is still an issue that requires ventilation. 253 Based on Kim et al. 254 assumption and calculations, the recirculation of air for 10 hours leads to approximately 20.12-20.174% oxygen ratio in the air, while the acceptable oxygen levels are between 19.5% and 23.5% (by volume), which allows the full recirculation of air within that period (10 hours).
A more efficient DAC system that captures from higher CO 2 concentrated streams (indoor air) was simulated by Zhao et al. 255 and the results showed that the optimal second law efficiencies for indoor CO 2 concentration of 3000 ppm, 2000 ppm and 1000 ppm are 44.57%, 37.55% and 31.6%, respectively. The effect of Indoor CO 2 concentration on indoor CO 2 lters was further investigated experimentally by Hu et al., 257 the results showed that the higher the inlet concentration (1000, 1500, 2000), the steeper the breakthrough curve, which signies a higher CO 2 capture rate. The above studies show the potential benets of the higher indoor CO 2 levels on DAC unit performance. Applying cooler CO 2 adsorption and its effects on DAC unit performance was further studied by Zhao et el., 255 the results showed an increase in second law efficiency from 15.38% to 39.41% when adsorption temperature changed from 323 to 298 K under 3000 ppm CO 2 level conditions. It was explained that decreasing adsorption temperature increases the adsorbent CO 2 capacity and reduces the minimum separation work. The previous results showed the potential for energy savings by simply locating the DAC unit in the cold region of the HVAC stream.
Finally, the moisture swing approach in capturing and producing CO 2 is considered the least energy consuming technique among others. However, it needs more water compared to other regeneration techniques. 258 Shi et al. conducted an experiment on ve different samples, showing that the higher relative humidity of air decreases the equilibrium CO 2 concentration. 259 The previous study highlights the potential of integrating a moisture swing-based DAC system with the HVAC system as different air streams with different humidity already exist within the HVAC system. Bryan and Salamah proposed the integration of CO 2 collectors inside the HVAC system. The system is located to capture CO 2 from the exhaust stream before leaving the building as shown in Fig. 14. It uses ion-exchange resin sorbent and moisture swing to adsorb and regenerate CO 2 . The advantage of their proposed system is that it uses the HVAC fan-free energy and the higher concentration of CO 2 in the exhaust stream. Based on their calculation, the system can offset 0.1% of USA carbon emissions if the system is installed in 50% of the existing commercial buildings. 256

Modeling of DAC and HVAC integration benets
Different methods were used to evaluate the performance of using CO 2 capture system within the built environment. Gall et al. 251 conducted an experiment to evaluate the performance of indoor air cleaner and track the indoor CO 2 concentration aer using a CO 2 capture unit, based on the setup shown in Fig. 15, by measuring the variation of indoor CO 2 concentration with respect to time with and without the air cleaner. The indoor CO 2 concentration level (C room ) in mole CO 2 m −3 for a room that uses a CO 2 capture unit can be modeled by the below equation. (20) where, V room is the room volume (m 3 ), Q is the outdoor air ventilation rate through the room (m 3 s −1 ), C a is the concentration of CO 2 in outdoor air (molesCO 2 m −3 ), C room is the concentration of CO 2 in the room as it enters the reactor, Q f is the volumetric ow rate through the reactor (m 3 s −1 ), and E is the total CO 2 emission rate from occupants in the room (moles s −1 ). Q and E can be obtained based on the procedure described by Persily et al., 260 the boundary condition for the above equation is such that at t = 0, C room = 0.0163 molesCO 2 m −3 (or 400 ppm CO 2 ) or whatever the initial concentration of room is and C f,20 (mol CO 2 m −3 ) is the concentration of CO 2 in the ow exiting the reactor at the last node which can be calculated based on below equations Here, the index i ranges from 1 to 19, V f,i is the volume of reactor element i, C f,i is the concentration of CO 2 in the well-mixed reactor element i, M i is the mass of unreacted sorbent in element i. To solve the above equation, parameters such as y(carbonation yield) and k(reaction constant) and boundary conditions need to be identied based on experimental measurements. The carbonation yield or total removed CO 2 was calculated by measuring the CO 2 concentration at the inlet and outlet of the indoor air cleaner and determining the time-integrated difference between both. The calculated mass of absorbed CO 2 was divided by the initial mass of sorbent and converted to moles to determine the molar yield (y) in moleCO 2 /mole sorbent , for reaction constant (k) estimation, the sum of squared errors between measured and modeled values of C f was calculated based on the following formula Different meaningful values of K were used to model C f , and then the value that achieved the minimized SSE function was used for the calculation, and nally, the boundary conditions were applied.
Another study that presents a model to track CO 2 concentration but added the energy-saving calculations by raising the air recirculation ratio was conducted by Kim et al. 254 They carried out experimental and numerical modeling to analyze the effect of using a CO 2 capture device on indoor CO 2 concentration, allowable air recirculation ratios, and the energy-saving potentials. The new system consists of both AHU and CO 2 capture units, which was compared to the conventional ventilation system. The modelling was implemented based on two congurations, as shown in Fig. 16.
The required outdoor airow rate (V s ) is correlated with the increase in indoor CO 2 concentration (C g ) and CO 2 generation rate per person (N) with the following equation; While for the mechanically ventilated rooms, the equation will be; V represents the room volume, Q is the volume ow rate, C 0 is the outdoor CO 2 concentration C(t) is the indoor CO 2 concentration and G(t) is the CO 2 generation rate per person, both are functions of time. The integration of the previous equation with the assumption of Q, C 0 and G(t) to be constant lead to the following equation; By adding CO 2 capture unit to the case of air recirculation, the equation will be as follow; where A(t) is the rate of CO 2 adsorption. Based on the experimental work, an equation was derived with an assumption of 800 m 3 h −1 air ow rate and 1 kg adsorption capacity. The equation can calculate the amount of CO 2 adsorbed in ppm for certain time.
The cooling and heating loads also were calculated using the following formula; One more model was also presented by Baus and Nehr, 261 who investigated the possibility of attaching DAC unit to the HVAC system as shown in Fig. 17. Experimental measurements were conducted for four different buildings in Germany and the data was used in a numerical model to assess the building CO 2 budget and potential energy saving by increasing air recirculation.
Based on the four evaluated buildings, the following equations were used to estimate building CO 2 load and potential energy saving to be 114.3-530.2 kgCO 2 per day and 0.34-2.57 MW h per ton. Where m i is the mass of either CO 2 or H 2 O in (g), t i is the time in (h), K i 1 is the emission rate due to occupancy (g h −1 ), K i 2 is the rate increasing due to ambient air feed (g h −1 ), K i 3 is the rate change from recycled air, K i 4 is air exchange rate in (1/h), K i 5 is the performed rate of CO 2 capture or dehumidication, _ Q AC is the energy consumption in HVAC, p air,hum is the specic density of humid air, _ V feed is the ambient air rate (m 3 h −1 ), C p,amb is the specic heat of ambient air, DT is the temperature difference between the design point and the ambient temperature, Q v is water enthalpy of vaporization and r H 2 O is the absolute humidity of air.
The effect of both CO 2 concentration level and cooler adsorption was modeled by a thermodynamic model presented by Zhao et al. 255 The model describes the performance of TVSA based indoor CO 2 capture unit and can be used as a method of comparison between different capturing technologies. It combines the thermodynamic second law of efficiency calculation, adsorption isotherms, and regeneration energy. The model can assess the effect of range of parameters such as adsorption temperature (T ads ), desorption temperature (T des ), desorption pressure (P des ), indoor CO 2 level (feed gas CO 2 concentration x A,1 ), CO 2 recovery and produced CO 2 purity on DAC thermodynamic efficiency (h TVSA ). The calculation can be performed using the following equations; where T H is the temperature of heat source, T L is the temperature of cooling source, T 0 is the ambient temperature and W min is the minimum separation work, which can be calculated as follow; where W ac is the actual work done which, is divided into the work done for pressurization W pump , input heat per cycle Q H and adsorption cooling energy per cycle Q L . Q H and Q L can be calculated for TVSA cycle using the below formulas based on assumed single gas adsorption (CO 2 ) by treating other components as inert gases, equilibrium capacity is reached, uniform temperature and CO 2 concentration in the sorbent are attained.
where q max is the maximum equilibrium CO 2 loading in the TVSA cycle; q min is the minimum equilibrium CO 2 adsorption amount in the TVSA cycle, while (DH(q)) is the desorption enthalpy and (q) is the adsorption capacity, which is calculated from the isotherm curves that change from material to another. The above model used amine-functionalized cellulose and its isotherm curves parameters and calculations were obtained from ref. 163 Other models available in the literature, which are associated with lter design and adsorption kinetics were conducted by Hu et al. 257 In the study, they ran an experiment to maintain acceptable IAQ levels by capturing excess CO 2 . Activated carbon lter impregnated with magnesium and calcium oxide was used as the capturing material. The lter performance was assessed based on initial removal efficiency, h 0 and pressure drop, which can be calculated based on Ergun equation with the interpretation of each parameter in ref. 257 et al.
The results showed that impregnated activated carbon with MgO promotes better adsorption performance than CaO. Moreover, the experiment and model showed that the initial efficiency increases as the air velocity decreases; however, the lower air velocity leads to a higher pressure drop, indicating that both parameters should be optimized for the best performance. Lee et al. 262 investigated the use of amine-modied Y-type zeolite in CO 2 adsorption and desorption cycles under indoor environmental conditions. Three types of amines were impregnated in the zeolite (TEPA, MEA and IPA), and TEPA achieved the highest capacity of 158 mg g −1 -adsorbent per hour under indoor CO 2 concentration of 1000 ppm. 262 The experimentally obtained concentrations of CO 2 with respect to time were successfully tted to the rst-order kinetic model and it was used to calculate the rate constants k. The highest k value between the three amines was achieved by TEPA.

Economic analysis of DAC and HVAC integration
Across different studies in the literature, it has been found that the key parameters affecting the capital and operating costs of DAC solid sorbent are the sorbent working capacity, cycle time, sorbent lifetime, desorption temperature and vacuum pressure. 79 Many of the economic assessments performed on DAC in the literature show varied results due to differences in assumptions and methodologies; these can make it difficult for direct comparisons. 80 The cost for current large-scale DAC systems ranges roughly between $80 per tCO 2 to $1133 per tCO 2 , and in the future it is expected to drop to around $34 to $260 per tCO 2 . 263 The improvements in contactor designs, sorbent properties, and heat integration are some of the parameters predicted to lower economic and environmental impacts. 263 Bioenergy with carbon capture and sequestration (BECCS) is a competitor for DAC and its cost ranges between $20-100$ per tCO 2 which is lower than that of DAC. Coastal blue carbon and terrestrial carbon removal are other alternatives with lower costs around $0-20 per tCO 2 . However, these technologies have drawbacks such as available land area, demand for wood and forestry management. 70 Capital expenditures (CAPEX), operating expenditures (OPEX) and the sorbent are the main factors that affect the overall cost of DAC. The type of sorbent selected can also affect the overall cost with liquid sorbents costing less. Climeworks that uses a solid sorbent has an overall cost of $600 per tCO 2 . 160 CAPEX contributes to most of the overall cost for this technology. Individual equipment components for the process have costs ranging from $0.13-420 million dollars. 70 OPEX such as maintenance, labour, waste removal and makeup are necessary to keep the process running. For both liquid and solid sorbents, the OPEX is below $100 per tCO 2 with solid systems having lower OPEX around $5 to $50 per tCO 2 . 238 Therefore, it is necessary to conduct research to reduce costs for a more economically favourable process. Since energy is required for heating, the source of energy also inuences the cost as well as the environmental impacts. Fossil fuel-based energy sources are cheaper but have a higher impact on the environment. Solar energy results in the cost being around $430 per tCO 2 , which is much higher than a fossil fuel based energy source such as coal. 264 Since solid adsorbent systems require regeneration temperatures of around 80-120°C, low-grade waste heat can be used. 265 DAC coupled with HVAC can result in reduced energy requirements for ventilation systems. Additionally, when integrating with existing HVAC systems, there are available blower (fans) and existing system (duct, utilities) to use for potentially better CO 2 capture at low temperatures. This should reduce the CAPEX for DAC as they are already available. Ji et al. 266 coupled DAC with a building air conditioning system in a study, which signicantly reduced the heat demand resulting in lower OPEX. They also found that not much additional cost was needed for retrot. In a theoretical study, Baus and Nehr 261 found that coupling DAC with HVAC in recirculation can potentially lower the energy requirements in buildings; resulting in lower operation costs. It was also highlighted that CO 2 absorbers can be expensive and can have stability issues in the long run needing replacement. Moreover, sorbents are exposed to thermal and mechanical stress in addition to reactive chemicals, which can lower the lifetime of the sorbents. Therefore, the technical feasibility of HVAC/DAC depends on sorbents with long lifetimes and affordable costs. The economic viability is quantied by the energy saving potential of HVAC/DAC integration in recirculation mode, which lowers OPEX. Therefore, the success of the HVAC/DAC coupling depends on the DAC technology itself and the effectiveness of integration of DAC into the energy infrastructure of the building. Even though the DAC technology is in its infancy, coupling with HVAC in buildings provides an energy saving potential, which can encourage the technology to enter the mass market. 261 Economic assessment methodology of DAC and HVAC integration. In this analysis, the cost assessment has been done based on a learning-by-doing model provided by Young et al. 267 for a DAC system. They found that DAC combined with renewable energy can breakeven by 2030. However, with combined cycle gas turbine (CCGT) energy, they found that the system was not economically viable and needed more energy efficiency. Baus and Nehr 261 simulated a model to assess the energy-saving potential of DAC/HVAC-coupling. They found the energy-saving potential with HVAC/DAC-coupling in recirculation mode to be between 7.85 and 52.97 MW h year −1 due to enhanced cooling for four different scenarios.
The economic model by Young et al. 267 has been modied in this paper to nd the economic viability of DAC/HVAC-coupling considering the energy-saving potential of cooling. The steps taken to carry out the simplistic analysis are mentioned in Fig. 18. The parameters for the economic assessment are kept the same as in the model by Young et al. 267 and the costs for the energy saving are modied. The energy used in the model is provided by a combined gas cycle turbine (CCGT). The main equation used by Young et al. 267 for the assessment is: where c d is plant construction cost, CO&M is the operation and maintenance cost. The operations and maintenance consists of the absorbent/adsorbent cost c a and the cost of energy c e and other operations/maintenance c o , and the cost of CO 2 Fig. 18 Methodology chart. processing is c p . Table 9 shows some of the input parameters used by Young et al. 267 The same parameters will be considered for this study and the energy requirements will be modied. Moreover, several economic indicators such as the net present value (NPV), internal rate of return (IRR) and payback period are calculated. The net present value (NPV) is calculated by: NPV ¼ X n t¼1 R t ð1 þ iÞ t À initial investment (45) where R t is the net present value, i is the discount rate, t is the time of cash ow and n is the time period. The internal rate of return (IRR) is calculated by: where R t is the net present value, i is the discount rate and t is the time of cash ow and n is the time period.
The following assumptions are undertaken to conduct the economic analysis: All costs for the plant construction, operation and maintenance are kept the same as in Young et al. 267 except the costs for energy requirement in the operations.
It is assumed that the retrotting costs are included in the existing model Young et al. 267 in the plant construction cost.
The CO 2 captured is to be sold at the carbon price of $40 per tCO 2 in 2020, peaking at $200 per tCO 2 in 2030 and dropping to 120$ per tCO 2 in 2050 to make the calculation simpler. Assuming linear changes between the prices. 52.97 MW h year −1 energy was saved by DAC/HVAC coupling in comparison to regular DAC 261 Discount rate of 7% is used for the NPV. All CO 2 captured is sold. The input parameters used in the economic modelling are given in Table 9.
Economic results and discussion for DAC and HVAC integration. The NPV was found to be $3047 per tCO 2 aer 50 years when the energy saving potential was assumed to be the highest at 52.97 MW h year −1 . Fig. 19 shows that the project would reach payback period in around 3 years from the start at the point where the line crosses the origin. Aer that, the graph shows a positive slope showing favorable economic conditions in the future. Moreover Fig. 20 shows a graph of NPV vs. incremental discount rate showing the IRR to be around 0.06 (6%). Young et al. 267 shows similar positive results for their cases 2-4.
Energy-saving potential of 52.97 MW h year −1 by Baus and Nehr 261 was for a less efficient AC; therefore, for a more modern AC unit, the energy-saving potential might be less. Hence, Fig. 21 shows how the energy-saving potential of DAC/HVACcoupling can affect the NPV. For lower energy saving potentials, the project would not be viable using CCGT energy and renewable sources of energy would need to be used as also suggested by Young et al. 267 Overall, lower energy costs and higher carbon costs would make the HVAC/DAC-coupling unit more economically favorable. As the cost of renewable energy reduces in the future, incorporating solar, wind, and geothermal technologies with DAC/HVAC could lead to a sustainable and cost-effective solution to reducing CO 2 levels in the atmosphere. Daniel et al. 268 performed a technoeconomic analysis on DAC carbon capture. The study found the NPV aer 50 years to range from 2 billion $ to around 24 billion$. This equates to around 82-1000 $ per tCO 2 and the highest NPV value is shown as a comparison with this study in green in Fig. 21. The NPV for 52.98 MW h year −1 energy saved in this study is higher than the literature; however, with lower energy recovery in the HVAC/DAC unit the results are comparable. Therefore, coupling DAC with HVAC can be economically favorable in comparison to only DAC. Additionally, HVAC and DAC coupling have other advantages such as improved air quality due to CO 2 reduction, energy savings due to increased air circulation and increased efficiency.

Challenges and future recommendation in DAC-HVAC integration
The HVAC and DAC integration brings complications as well as potential savings. These complications apply to some challenges that can be explained in the following points. For energy efficiency purposes, full air recirculation is recommended, but the build-up of VOC requires additional lters, which increases the pressure drop in the air supply stream.
The integration of DAC unit with the HVAC systems implies additional pressure drop in the supply air stream, which requires CO 2 lter/sorbent material geometry's optimization.
For the CO 2 lter to achieve building negative emission concept, the effluent air CO 2 level should be lower than the ambient CO 2 level, as a result, the minimum acceptable indoor CO 2 level should be investigated.
The lter location should be positioned in the coldest stream; however, the coldest stream would have the highest relative humidity. The humid air will cause water adsorption and brings more energy consumption for water regeneration. This discussion raises a research challenge to nd material that co-adsorb the least water molecules.
It is important for future research in DAC-HVAC integration to utilize the cold stream for adsorbent cooling aer temperature-dependent regeneration.
For recommendations, HVAC indoor environment requires the use of environmentally friendly materials (sorbents) in order to safeguard the health of the occupants.
Finally, investigating a way to combine humidity swings in HVAC and moisture swing adsorption of CO 2 without mixing the produced CO 2 with air again is recommended.

Conclusions
This work has reviewed the literature on emerging trends in direct air capture of CO 2 . The discussion is directed at the main drivers (sorbent systems and regeneration options) of DAC for commercialization. It is well established that a good sorbent should have high CO 2 selectivity, capacity, free binding energy, low regeneration energy, thermal stability, and cyclic stability. The two main capture options in DAC, the liquid and solid sorbents, have been tested for these characteristics. In general, the liquid sorbents are volatile, which implies heat loss due to evaporation and have lower kinetics compared to solid sorbents. For the aqueous solution of metal hydroxides, they need high temperature (up to 900°C) for regeneration, although amine solutions have been reported with lower regeneration temperature. For solid sorbents, especially the physisorption materials, the major issue that affects their performance is their strong affinity for moisture in the atmosphere. If the strong water affinity issue is resolved, MOFs nanocomposite and porous frameworks are good sorbent candidates as they have high selectivity towards CO 2 over N 2 and other gases in the atmosphere. For chemisorption materials, depending on the category, the major issue affecting their performance is poor cyclic stability, which leads to leaching of amines during operations, and the poor cyclic stability is compounded when water is co-adsorbed. Although covalently bound polymeric amines on solid supports that are known as hyper-branched amino silica (HAS) have been able to solve this issue as reported. HAS offer high capacity, stability, easy preparation, low cost, and excellent regeneration compared to other categories of chemisorption materials.
As for the energy requirements for DAC system if high purity CO 2 is desired, it should be noted that there is no signicant difference in the required energy between absorption and adsorption systems. More precisely, the thermal and electrical energy requirements for solid sorbent are 3-6 GJ tCO 2 −1 and 1.5 GJ tCO 2 −1 , respectively. While for liquid sorbent, the thermal and electrical energy requirements are 5.25-8.1 GJ tCO 2 −1 and 1.3-1.8 GJ tCO 2 −1 , respectively, depending on the contactor conguration and packing materials. However, moisture swing adsorption (MSA) offers opportunities if high purity CO 2 is not desired, because it has low energy consumption compared to other methods. The mechanism of MSA opens up potentials for integrating DAC with other systems such as HVAC for creating DAC system with minimum energy requirements. As illustrated in the economic analysis of DAC-HVAC integration, research efforts are already looking promising in DAC-HVAC integration; however, health impact of the sorbents to be used in HVAC systems needs to be investigated because of the sensitivity of the HVAC systems to human wellbeing. Investigation of the suitable sorbents, regeneration method, lter position, and lter design is required to gain the most of the proposed integration.